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');
   });
 });

src/frontends/tui/commands.ts

@@ -5,6 +5,7 @@ export type Command =
   | { type: 'status' }
   | { type: 'fullscreen' }
   | { type: 'compact' }
+  | { type: 'usage' }
   | { type: 'model'; name?: string }
   | { type: 'backend'; provider?: string }
   | { type: 'transfer'; target: string }
@@ -44,6 +45,11 @@ export function parseCommand(input: string): Command | null {
     return { type: 'compact' };
   }
 
+  // Usage
+  if (trimmed === '/usage') {
+    return { type: 'usage' };
+  }
+
   // Model (with optional argument)
   if (trimmed === '/model') {
     return { type: 'model' };
@@ -80,6 +86,7 @@ Commands:
   /backend [provider]    Show or switch local backend (ollama, llamacpp)
   /reset, /clear, /new   Clear conversation history
   /compact               Compact conversation history
+  /usage                 Show token usage and estimated cost
   /status                Show session info and token usage
   /fullscreen, /fs       Switch to fullscreen mode
   /transfer <dest>       Transfer session to another frontend
@@ -98,6 +105,7 @@ export const SLASH_COMMANDS = [
   '/clear',
   '/new',
   '/compact',
+  '/usage',
   '/status',
   '/fullscreen',
   '/fs',
@@ -115,6 +123,7 @@ export const COMMAND_TOOLTIPS: Record<string, string> = {
   '/clear': 'Clear conversation history',
   '/new': 'Start a new conversation',
   '/compact': 'Compact conversation history to save context space',
+  '/usage': 'Show token usage and estimated cost',
   '/status': 'Show session info and token usage',
   '/fullscreen': 'Switch to fullscreen mode',
   '/fs': 'Switch to fullscreen mode',