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/frontends/tui/commands.test.ts

@@ -30,6 +30,10 @@ describe('parseCommand', () => {
     expect(parseCommand('/compact')).toEqual({ type: 'compact' });
   });
 
+  it('parses /usage command', () => {
+    expect(parseCommand('/usage')).toEqual({ type: 'usage' });
+  });
+
   it('parses /model command without argument', () => {
     expect(parseCommand('/model')).toEqual({ type: 'model' });
   });
@@ -69,6 +73,7 @@ describe('getHelpText', () => {
     expect(help).toContain('/model');
     expect(help).toContain('/reset');
     expect(help).toContain('/compact');
+    expect(help).toContain('/usage');
     expect(help).toContain('/quit');
   });
 });