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/utils.ts

@@ -0,0 +1,33 @@
+/**
+ * Shared utilities for channel adapters.
+ */
+
+/**
+ * Split a long message into chunks that respect a platform's character limit.
+ * Prefers splitting at newlines, then spaces, then hard-cuts.
+ */
+export 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;
+}