refactor(channels,router): move all policy to router; bridge is transport

Follow-up to b159722. That shrank the bridge's shouldEngage to a flood
gate + coarse sticky-subscribe signal. This completes the move —
policy lives exclusively in the router, the bridge is transport-only,
and the conversations map + ChannelSetup.conversations +
ChannelAdapter.updateConversations are all gone.

Key shifts:

1. Subscribe moves from bridge to router.

   Bridge used to call `thread.subscribe()` from its onNewMention /
   onDirectMessage handlers based on a coarse "any mention-sticky wiring
   exists on this channel" check. That forced the decision before the
   router could apply per-wiring engage logic, and it relied on the
   conversations map being current (staleness risk).

   ChannelAdapter gains `subscribe?(platformId, threadId)`. The Chat
   SDK bridge implements it via SqliteStateAdapter.subscribe(threadId)
   (idempotent — a repeat call on an already-subscribed thread is a
   no-op). The router's fan-out loop calls it once per message when
   the first mention-sticky wiring actually engages. Precise, not
   coarse.

2. Short-circuit the drop path with one combined query.

   New `getMessagingGroupWithAgentCount(channelType, platformId)` does
   the messaging_groups lookup AND counts wirings in a single SELECT,
   using the existing UNIQUE(channel_type, platform_id) index on
   messaging_groups and UNIQUE(messaging_group_id, agent_group_id) on
   messaging_group_agents for the JOIN. No new indexes needed.

   routeInbound now short-circuits:
     - No messaging_groups row AND not addressed (no mention/DM)
       → return silently. One DB read, nothing written. This is the
       Discord-bot-in-a-big-guild case; we no longer auto-create rows
       for every plain message in every channel the bot can see.
     - Messaging group exists but no wirings AND not addressed
       → return silently. One DB read.
     - Otherwise fall through to sender resolution + fan-out as before.

   Behavioral change: plain chatter on unwired channels no longer gets
   dropped_messages audit rows, which used to bloat the table. Audit
   still fires on addressed-to-bot drops where the admin cares
   ("someone @-mentioned us but nobody's wired").

3. Bridge is now purely transport.

   Deleted entirely: ConversationConfig, ChannelSetup.conversations,
   ChannelAdapter.updateConversations?, bridge's `conversations` map,
   buildConversationMap, shouldEngage, EngageSource, engageDecision,
   bridge.updateConversations method, src/index.ts
   buildConversationConfigs. Four handlers reduce to "resolve channel
   id, build InboundMessage with isMention, call onInbound". Net
   ~130 LOC deleted from the bridge.

   Collateral: the conversations-map staleness problem is gone. The
   upcoming channel-registration feature doesn't need any map-refresh
   plumbing — when an approval creates a new wiring, the next message
   hits the DB fresh and just works.

Bridge tests prune to the narrow platform-adjacent surface (openDM
delegation, subscribe presence). Host-core test that asserted the
old "auto-create on every unknown message" behavior updates to
reflect the new escalation-gated semantics: plain messages on
unknown channels don't auto-create, mentions do.

159 tests pass (was 172 — net -13, almost entirely from
bridge-engage-mode tests that covered logic now owned by the router
and exercised through host-core.test.ts).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.ts

@@ -9,7 +9,6 @@ import path from 'path';
 import { DATA_DIR } from './config.js';
 import { initDb } from './db/connection.js';
 import { runMigrations } from './db/migrations/index.js';
-import { getMessagingGroupsByChannel, getMessagingGroupAgents } from './db/messaging-groups.js';
 import { ensureContainerRuntimeRunning, cleanupOrphans } from './container-runtime.js';
 import { startActiveDeliveryPoll, startSweepDeliveryPoll, setDeliveryAdapter, stopDeliveryPolls } from './delivery.js';
 import { startHostSweep, stopHostSweep } from './host-sweep.js';
@@ -52,7 +51,7 @@ import './channels/index.js';
 // append registry-based modules. Imported for side effects (registrations).
 import './modules/index.js';
 
-import type { ChannelAdapter, ChannelSetup, ConversationConfig } from './channels/adapter.js';
+import type { ChannelAdapter, ChannelSetup } from './channels/adapter.js';
 import { initChannelAdapters, teardownChannelAdapters, getChannelAdapter } from './channels/channel-registry.js';
 
 async function main(): Promise<void> {
@@ -70,9 +69,7 @@ async function main(): Promise<void> {
 
   // 3. Channel adapters
   await initChannelAdapters((adapter: ChannelAdapter): ChannelSetup => {
-    const conversations = buildConversationConfigs(adapter.channelType);
     return {
-      conversations,
       onInbound(platformId, threadId, message) {
         routeInbound({
           channelType: adapter.channelType,
@@ -151,28 +148,6 @@ async function main(): Promise<void> {
   log.info('NanoClaw running');
 }
 
-/** Build ConversationConfig[] for a channel type from the central DB. */
-function buildConversationConfigs(channelType: string): ConversationConfig[] {
-  const groups = getMessagingGroupsByChannel(channelType);
-  const configs: ConversationConfig[] = [];
-
-  for (const mg of groups) {
-    const agents = getMessagingGroupAgents(mg.id);
-    for (const agent of agents) {
-      configs.push({
-        platformId: mg.platform_id,
-        agentGroupId: agent.agent_group_id,
-        engageMode: agent.engage_mode,
-        engagePattern: agent.engage_pattern,
-        ignoredMessagePolicy: agent.ignored_message_policy,
-        sessionMode: agent.session_mode,
-      });
-    }
-  }
-
-  return configs;
-}
-
 /** Graceful shutdown. */
 async function shutdown(signal: string): Promise<void> {
   log.info('Shutdown signal received', { signal });