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>

src/db/schema.ts

@@ -76,7 +76,7 @@ CREATE TABLE pending_questions (
  *   outbound.db — container writes, host reads (read-only open)
  */
 
-/** Host-owned: inbound messages + delivery tracking. */
+/** Host-owned: inbound messages + delivery tracking + destination map. */
 export const INBOUND_SCHEMA = `
 CREATE TABLE messages_in (
   id             TEXT PRIMARY KEY,
@@ -101,6 +101,19 @@ CREATE TABLE delivered (
   status              TEXT NOT NULL DEFAULT 'delivered',
   delivered_at        TEXT NOT NULL
 );
+
+-- Destination map for this session's agent.
+-- Host overwrites on every container wake AND on demand (admin rewires, new child agents, etc.).
+-- Container queries this live on every lookup, so admin changes take effect
+-- mid-session without requiring a container restart.
+CREATE TABLE destinations (
+  name            TEXT PRIMARY KEY,
+  display_name    TEXT,
+  type            TEXT NOT NULL,   -- 'channel' | 'agent'
+  channel_type    TEXT,            -- for type='channel'
+  platform_id     TEXT,            -- for type='channel'
+  agent_group_id  TEXT             -- for type='agent'
+);
 `;
 
 /** Container-owned: outbound messages + processing acknowledgments. */