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/discord/adapter.ts

@@ -15,6 +15,7 @@ import type {
   ChannelAdapter,
   ChannelStatus,
 } from '../types.js';
+import { splitMessage } from '../utils.js';
 
 /** Configuration for the Discord channel adapter. */
 export interface DiscordAdapterConfig {
@@ -27,36 +28,6 @@ export interface DiscordAdapterConfig {
   requireMention?: boolean;
 }
 
-/**
- * Split a long message into chunks that respect Discord's 2000 char limit.
- * Prefers splitting at newlines, then spaces, then hard-cuts.
- */
-function splitMessage(text: string, maxLength: number): string[] {
-  const chunks: string[] = [];
-  let remaining = text;
-
-  while (remaining.length > 0) {
-    if (remaining.length <= maxLength) {
-      chunks.push(remaining);
-      break;
-    }
-
-    // Try to split at a newline within the allowed window
-    let splitIndex = remaining.lastIndexOf('\n', maxLength);
-    if (splitIndex === -1 || splitIndex < maxLength / 2) {
-      splitIndex = remaining.lastIndexOf(' ', maxLength);
-    }
-    if (splitIndex === -1 || splitIndex < maxLength / 2) {
-      splitIndex = maxLength;
-    }
-
-    chunks.push(remaining.slice(0, splitIndex));
-    remaining = remaining.slice(splitIndex).trimStart();
-  }
-
-  return chunks;
-}
-
 /**
  * Discord channel adapter backed by discord.js.
  *