refactor: move destinations from JSON file into inbound.db

The per-session destination map was being written as a sidecar JSON file
(/workspace/.nanoclaw-destinations.json) — inconsistent with the rest of
v2, where all host↔container IO goes through inbound.db / outbound.db.

Move it into a `destinations` table in INBOUND_SCHEMA. The host writes
it before every container wake AND on demand (e.g. after create_agent)
so the creator sees the new child destination mid-session without a
restart. The container queries the table live on every lookup — no
cache, no staleness window.

- src/db/schema.ts: add `destinations` table to INBOUND_SCHEMA.
- src/session-manager.ts: writeDestinationsFile → writeDestinations,
  writes via DELETE + INSERT inside a transaction.
- src/delivery.ts: create_agent handler calls writeDestinations on the
  creator's session after inserting the new destination rows.
- container/agent-runner/src/destinations.ts: queries inbound.db
  directly in every findByName/getAllDestinations/findByRouting call.
  No more cache. No setDestinationsForTest (obsolete). No fs import.
- container/agent-runner/src/index.ts and mcp-tools/index.ts: remove
  loadDestinations() calls — no longer needed.
- Test helper initTestSessionDb creates the destinations table.
  Integration test inserts a row directly instead of mocking the cache.

No backwards compatibility: sessions predating the schema update must
be recreated. This is fine on the v2 branch.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/agent-runner/src/index.ts

@@ -26,7 +26,7 @@ import fs from 'fs';
 import path from 'path';
 import { fileURLToPath } from 'url';
 
-import { buildSystemPromptAddendum, loadDestinations } from './destinations.js';
+import { buildSystemPromptAddendum } from './destinations.js';
 import { createProvider, type ProviderName } from './providers/factory.js';
 import { runPollLoop } from './poll-loop.js';
 
@@ -45,9 +45,6 @@ async function main(): Promise<void> {
 
   const provider = createProvider(providerName, { assistantName });
 
-  // Load destination map (written by host on every wake)
-  loadDestinations();
-
   // Load global CLAUDE.md as additional system context, then append destinations addendum
   let systemPrompt: string | undefined;
   if (fs.existsSync(GLOBAL_CLAUDE_MD)) {