feat: add P2 features — retry policy, prompt templating, usage tracking, tech debt cleanup

- Extract shared splitMessage() into channels/utils.ts (dedup 4 adapters)
- Add Slack user name resolution with caching (users.info API)
- Add withRetry() with exponential backoff + jitter, isRetryable() filter
- Wire retry config into ModelRouter.chat() (non-streaming only)
- Add assembleSystemPrompt() multi-file template system (SOUL/AGENTS/IDENTITY/USER/TOOLS.md)
- Add usage tracking accumulators in NativeAgent + AgentOrchestrator
- Add estimateCost() with per-model pricing table
- Add /usage TUI command with full usage report formatting
- Add retrySchema and promptSchema to config schema

Tests: 569 passing, typecheck clean

src/prompt/index.ts

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+export { assembleSystemPrompt, type PromptTemplateConfig, type PromptTemplateResult } from './template.js';

src/prompt/template.test.ts

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+import { describe, it, expect, afterEach } from 'vitest';
+import { mkdtempSync, writeFileSync, rmSync } from 'fs';
+import { join } from 'path';
+import { tmpdir } from 'os';
+import { assembleSystemPrompt } from './template.js';
+
+describe('assembleSystemPrompt', () => {
+  const tempDirs: string[] = [];
+
+  function makeTempDir(): string {
+    const dir = mkdtempSync(join(tmpdir(), 'flynn-prompt-test-'));
+    tempDirs.push(dir);
+    return dir;
+  }
+
+  afterEach(() => {
+    for (const dir of tempDirs) {
+      rmSync(dir, { recursive: true, force: true });
+    }
+    tempDirs.length = 0;
+  });
+
+  it('returns fallback when no files found', () => {
+    const dir = makeTempDir();
+    const result = assembleSystemPrompt({ searchDirs: [dir] });
+
+    expect(result.prompt).toBe(
+      'You are Flynn, a helpful personal AI assistant. Be direct, concise, and helpful. Use markdown when it improves readability.',
+    );
+    expect(result.loadedFiles).toEqual([]);
+  });
+
+  it('loads SOUL.md without section header', () => {
+    const dir = makeTempDir();
+    writeFileSync(join(dir, 'SOUL.md'), 'You are Flynn.');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir] });
+
+    expect(result.prompt).toBe('You are Flynn.');
+    expect(result.loadedFiles).toHaveLength(1);
+    expect(result.loadedFiles[0]).toContain('SOUL.md');
+  });
+
+  it('loads AGENTS.md with "# Agent Instructions" section header', () => {
+    const dir = makeTempDir();
+    writeFileSync(join(dir, 'AGENTS.md'), 'Follow these rules.');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir] });
+
+    expect(result.prompt).toBe('# Agent Instructions\n\nFollow these rules.');
+    expect(result.loadedFiles).toHaveLength(1);
+    expect(result.loadedFiles[0]).toContain('AGENTS.md');
+  });
+
+  it('loads multiple files in correct order', () => {
+    const dir = makeTempDir();
+    writeFileSync(join(dir, 'SOUL.md'), 'I am Flynn.');
+    writeFileSync(join(dir, 'AGENTS.md'), 'Be helpful.');
+    writeFileSync(join(dir, 'USER.md'), 'User likes cats.');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir] });
+
+    expect(result.loadedFiles).toHaveLength(3);
+    // Verify correct ordering: SOUL → AGENTS → USER
+    expect(result.prompt).toBe(
+      'I am Flynn.\n\n# Agent Instructions\n\nBe helpful.\n\n# User Context\n\nUser likes cats.',
+    );
+  });
+
+  it('first search dir takes precedence over later dirs', () => {
+    const dir1 = makeTempDir();
+    const dir2 = makeTempDir();
+    writeFileSync(join(dir1, 'SOUL.md'), 'Primary identity.');
+    writeFileSync(join(dir2, 'SOUL.md'), 'Secondary identity.');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir1, dir2] });
+
+    expect(result.prompt).toBe('Primary identity.');
+    expect(result.loadedFiles).toHaveLength(1);
+    expect(result.loadedFiles[0]).toContain(dir1);
+  });
+
+  it('falls through to later dir if file missing in first dir', () => {
+    const dir1 = makeTempDir();
+    const dir2 = makeTempDir();
+    writeFileSync(join(dir2, 'SOUL.md'), 'Fallback identity.');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir1, dir2] });
+
+    expect(result.prompt).toBe('Fallback identity.');
+    expect(result.loadedFiles[0]).toContain(dir2);
+  });
+
+  it('extra sections are appended', () => {
+    const dir = makeTempDir();
+    writeFileSync(join(dir, 'SOUL.md'), 'Base identity.');
+
+    const result = assembleSystemPrompt({
+      searchDirs: [dir],
+      extraSections: [
+        { name: 'Custom Rules', content: 'Always be polite.' },
+      ],
+    });
+
+    expect(result.prompt).toBe(
+      'Base identity.\n\n# Custom Rules\n\nAlways be polite.',
+    );
+  });
+
+  it('empty files are skipped', () => {
+    const dir = makeTempDir();
+    writeFileSync(join(dir, 'SOUL.md'), '');
+    writeFileSync(join(dir, 'AGENTS.md'), '   ');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir] });
+
+    expect(result.prompt).toBe(
+      'You are Flynn, a helpful personal AI assistant. Be direct, concise, and helpful. Use markdown when it improves readability.',
+    );
+    expect(result.loadedFiles).toEqual([]);
+  });
+
+  it('empty extra sections are skipped', () => {
+    const dir = makeTempDir();
+    writeFileSync(join(dir, 'SOUL.md'), 'Base identity.');
+
+    const result = assembleSystemPrompt({
+      searchDirs: [dir],
+      extraSections: [
+        { name: 'Empty', content: '   ' },
+        { name: 'Populated', content: 'Has content.' },
+      ],
+    });
+
+    expect(result.prompt).toBe(
+      'Base identity.\n\n# Populated\n\nHas content.',
+    );
+  });
+
+  it('attempts all PROMPT_FILES', () => {
+    const dir = makeTempDir();
+    writeFileSync(join(dir, 'SOUL.md'), 'Soul.');
+    writeFileSync(join(dir, 'AGENTS.md'), 'Agents.');
+    writeFileSync(join(dir, 'IDENTITY.md'), 'Identity.');
+    writeFileSync(join(dir, 'USER.md'), 'User.');
+    writeFileSync(join(dir, 'TOOLS.md'), 'Tools.');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir] });
+
+    expect(result.loadedFiles).toHaveLength(5);
+    expect(result.prompt).toContain('Soul.');
+    expect(result.prompt).toContain('# Agent Instructions\n\nAgents.');
+    expect(result.prompt).toContain('# Identity Customization\n\nIdentity.');
+    expect(result.prompt).toContain('# User Context\n\nUser.');
+    expect(result.prompt).toContain('# Tool Instructions\n\nTools.');
+  });
+
+  it('trims whitespace from loaded file content', () => {
+    const dir = makeTempDir();
+    writeFileSync(join(dir, 'SOUL.md'), '\n  I am Flynn.  \n\n');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir] });
+
+    expect(result.prompt).toBe('I am Flynn.');
+  });
+
+  it('mixes files from different search directories', () => {
+    const dir1 = makeTempDir();
+    const dir2 = makeTempDir();
+    writeFileSync(join(dir1, 'SOUL.md'), 'Primary soul.');
+    writeFileSync(join(dir2, 'AGENTS.md'), 'Agent rules.');
+
+    const result = assembleSystemPrompt({ searchDirs: [dir1, dir2] });
+
+    expect(result.loadedFiles).toHaveLength(2);
+    expect(result.prompt).toBe('Primary soul.\n\n# Agent Instructions\n\nAgent rules.');
+  });
+});

src/prompt/template.ts

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+import { readFileSync, existsSync } from 'fs';
+import { resolve } from 'path';
+
+/** Ordered list of prompt template files to look for. */
+const PROMPT_FILES = [
+  { name: 'SOUL.md', section: 'Identity', required: false },
+  { name: 'AGENTS.md', section: 'Agent Instructions', required: false },
+  { name: 'IDENTITY.md', section: 'Identity Customization', required: false },
+  { name: 'USER.md', section: 'User Context', required: false },
+  { name: 'TOOLS.md', section: 'Tool Instructions', required: false },
+] as const;
+
+export interface PromptTemplateConfig {
+  /** Directories to search for template files, in priority order. */
+  searchDirs: string[];
+  /** Additional sections to inject (e.g., from config). */
+  extraSections?: Array<{ name: string; content: string }>;
+}
+
+export interface PromptTemplateResult {
+  /** The assembled system prompt. */
+  prompt: string;
+  /** Which files were loaded. */
+  loadedFiles: string[];
+}
+
+/**
+ * Assemble a system prompt from multiple template files.
+ *
+ * Searches `searchDirs` in order for each template file.
+ * First match wins — a file found in an earlier directory takes precedence.
+ * Sections are assembled in the order defined in PROMPT_FILES.
+ */
+export function assembleSystemPrompt(config: PromptTemplateConfig): PromptTemplateResult {
+  const sections: string[] = [];
+  const loadedFiles: string[] = [];
+
+  for (const { name, section } of PROMPT_FILES) {
+    for (const dir of config.searchDirs) {
+      const filePath = resolve(dir, name);
+      if (existsSync(filePath)) {
+        const content = readFileSync(filePath, 'utf-8').trim();
+        if (content) {
+          // SOUL.md is special — it's the base identity, no section header
+          if (name === 'SOUL.md') {
+            sections.push(content);
+          } else {
+            sections.push(`# ${section}\n\n${content}`);
+          }
+          loadedFiles.push(filePath);
+        }
+        break; // First match wins for this file
+      }
+    }
+  }
+
+  // Add extra sections
+  if (config.extraSections) {
+    for (const { name, content } of config.extraSections) {
+      if (content.trim()) {
+        sections.push(`# ${name}\n\n${content.trim()}`);
+      }
+    }
+  }
+
+  // Fallback if nothing was loaded
+  if (sections.length === 0) {
+    return {
+      prompt: 'You are Flynn, a helpful personal AI assistant. Be direct, concise, and helpful. Use markdown when it improves readability.',
+      loadedFiles: [],
+    };
+  }
+
+  return {
+    prompt: sections.join('\n\n'),
+    loadedFiles,
+  };
+}