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/template.ts

@@ -0,0 +1,78 @@
+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,
+  };
+}