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/channels/index.ts

@@ -7,6 +7,7 @@ export type {
   MessageHandler,
 } from './types.js';
 export { ChannelRegistry } from './registry.js';
+export { splitMessage } from './utils.js';
 export { TelegramAdapter, type TelegramAdapterConfig } from './telegram/index.js';
 export { WebChatAdapter, type WebChatAdapterConfig } from './webchat/index.js';
 export { DiscordAdapter, type DiscordAdapterConfig } from './discord/index.js';