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/backends/native/agent.ts

@@ -1,4 +1,4 @@
-import type { ModelClient, Message, ChatRequest, ChatResponse, ModelToolCall } from '../../models/types.js';
+import type { ModelClient, Message, ChatRequest, ChatResponse, ModelToolCall, TokenUsage } from '../../models/types.js';
 import type { ModelRouter, ModelTier } from '../../models/router.js';
 import type { Session } from '../../session/index.js';
 import type { ToolRegistry } from '../../tools/registry.js';
@@ -39,6 +39,8 @@ export class NativeAgent {
   private toolExecutor?: ToolExecutor;
   private maxIterations: number;
   private onToolUse?: (event: ToolUseEvent) => void;
+  private _totalUsage: TokenUsage = { inputTokens: 0, outputTokens: 0 };
+  private _callCount: number = 0;
 
   constructor(config: NativeAgentConfig) {
     this.modelClient = config.modelClient;
@@ -79,6 +81,10 @@ export class NativeAgent {
 
     const response = await this.chatWithRouter(request);
 
+    this._totalUsage.inputTokens += response.usage.inputTokens;
+    this._totalUsage.outputTokens += response.usage.outputTokens;
+    this._callCount++;
+
     if (response.fallback) {
       console.warn(`[Flynn] ${response.fallbackReason}`);
     }
@@ -110,6 +116,10 @@ export class NativeAgent {
 
       const response = await this.chatWithRouter(request);
 
+      this._totalUsage.inputTokens += response.usage.inputTokens;
+      this._totalUsage.outputTokens += response.usage.outputTokens;
+      this._callCount++;
+
       if (response.fallback) {
         console.warn(`[Flynn] ${response.fallbackReason}`);
       }
@@ -185,6 +195,16 @@ export class NativeAgent {
     } else {
       this.inMemoryHistory = [];
     }
+    this.resetUsage();
+  }
+
+  getUsage(): { inputTokens: number; outputTokens: number; calls: number } {
+    return { ...this._totalUsage, calls: this._callCount };
+  }
+
+  resetUsage(): void {
+    this._totalUsage = { inputTokens: 0, outputTokens: 0 };
+    this._callCount = 0;
   }
 
   getHistory(): Message[] {