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/db/messaging-groups.ts

@@ -37,6 +37,37 @@ export function getMessagingGroupByPlatform(channelType: string, platformId: str
     .get(channelType, platformId) as MessagingGroup | undefined;
 }
 
+/**
+ * Combined lookup for the router's fast-drop path. Returns the messaging
+ * group (if it exists) and a count of wired agents in one query — lets
+ * `routeInbound` short-circuit messages for unwired / unknown channels
+ * with a single DB read instead of four (mg lookup, sender upsert, agents
+ * lookup, dropped_messages insert).
+ *
+ * Returns `null` when no messaging_groups row exists for this channel.
+ * Returns `{ mg, agentCount: 0 }` when the row exists but has no wired
+ * agents. Uses the `UNIQUE(channel_type, platform_id)` index plus the
+ * `UNIQUE(messaging_group_id, agent_group_id)` index for the JOIN — both
+ * covered by existing SQLite auto-indexes from the UNIQUE constraints.
+ */
+export function getMessagingGroupWithAgentCount(
+  channelType: string,
+  platformId: string,
+): { mg: MessagingGroup; agentCount: number } | null {
+  const row = getDb()
+    .prepare(
+      `SELECT mg.*, COUNT(mga.id) AS agent_count
+         FROM messaging_groups mg
+    LEFT JOIN messaging_group_agents mga ON mga.messaging_group_id = mg.id
+        WHERE mg.channel_type = ? AND mg.platform_id = ?
+     GROUP BY mg.id`,
+    )
+    .get(channelType, platformId) as (MessagingGroup & { agent_count: number }) | undefined;
+  if (!row) return null;
+  const { agent_count, ...mg } = row;
+  return { mg: mg as MessagingGroup, agentCount: agent_count };
+}
+
 export function getAllMessagingGroups(): MessagingGroup[] {
   return getDb().prepare('SELECT * FROM messaging_groups ORDER BY name').all() as MessagingGroup[];
 }