feat: add seq cursor, work_items, provider fallback, and delivery reliability

Major reliability improvements to the message processing pipeline:

- Add messages.seq monotonic cursor replacing timestamp-based cursors,
  preventing message loss from timestamp collisions with LIMIT queries
- Add work_items table separating agent production from delivery,
  enabling delivery retry without re-running the agent
- Propagate Discord send failures instead of silently swallowing them
- Add Claude 429 → Kimi K2.5 automatic provider fallback with cooldown
- Fix follow-up turn state reset in live index.ts path (not just
  message-runtime.ts) to prevent final output from being lost
- Add restart context tracking for graceful restart announcements
- Lazy migration from timestamp cursors to seq cursors for existing data

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -7,6 +7,7 @@
   "scripts": {
     "build": "tsc",
     "build:runners": "npm --prefix runners/agent-runner install && npm --prefix runners/agent-runner run build && npm --prefix runners/codex-runner install && npm --prefix runners/codex-runner run build",
+    "restart:hint": "tsx src/restart-context-cli.ts write",
     "start": "node dist/index.js",
     "dev": "tsx src/index.ts",
     "typecheck": "tsc --noEmit",