feat(routing): engage modes + sender scope + accumulate/drop + per-agent fan-out

Replaces the opaque trigger_rules JSON + response_scope enum on
messaging_group_agents with four explicit orthogonal columns:

    engage_mode            'pattern' | 'mention' | 'mention-sticky'
    engage_pattern         regex source; required when mode='pattern';
                           '.' is the "always" sentinel
    sender_scope           'all' | 'known'
    ignored_message_policy 'drop' | 'accumulate'

Inbound routing becomes a fan-out — every wired agent is evaluated
independently. A match gets its own session + container wake. A miss
with accumulate keeps the message as context-only (trigger=0) in that
agent's session, so when the agent does eventually engage it sees the
prior chatter.

## Schema

- Migration 010 (`engage-modes`): adds the 4 new columns, backfills
  from trigger_rules.pattern + requiresTrigger + response_scope, drops
  the legacy columns.
- messages_in gains `trigger INTEGER NOT NULL DEFAULT 1` (session DB
  schema + `migrateMessagesInTable` forward-compat).
- countDueMessages gates waking on `trigger = 1`.

## Routing

- `pickAgent` (returns one) → loop over all wired agents. Per agent:
  evaluate engage_mode; run access gate + sender-scope gate; on full
  match → resolveSession + writeSessionMessage(trigger=1) + wake. On
  miss with accumulate → writeSessionMessage(trigger=0), no wake. On
  miss with drop → skip.
- New `findSessionForAgent(agentGroupId, mgId, threadId)` scopes
  session lookup by agent so fan-out doesn't cross sessions.
- `messageIdForAgent` namespaces inbound message ids by agent_group_id
  so PRIMARY KEY doesn't collide across per-agent session DBs.

## Adapter layer

- `ConversationConfig` replaces `triggerPattern` + `requiresTrigger`
  with `engageMode` + `engagePattern`.
- Chat SDK bridge stores `Map<platformId, ConversationConfig[]>` (multi-
  agent per conversation) and applies union gating pre-onInbound:
    * onSubscribedMessage: engage if any wiring keeps firing in
      subscribed state (mention-sticky or pattern)
    * onNewMention: engage on mention; only subscribes the thread if
      at least one wiring is `mention-sticky`
    * onDirectMessage: engage per mode; sticky follows same rule
- Bridge no longer unconditionally calls `thread.subscribe()`.

## Sender scope

- Permissions module registers a second hook `setSenderScopeGate` that
  runs per-wiring after the existing access gate. `sender_scope='known'`
  requires canAccessAgentGroup(); `'all'` is a no-op. Not installed →
  no-op everywhere (default allow).

## Container side

- Host passes `NANOCLAW_MAX_MESSAGES_PER_PROMPT` (reuses existing
  MAX_MESSAGES_PER_PROMPT config; was dead code from v1).
- `getPendingMessages` queries `ORDER BY seq DESC LIMIT N`, reverses to
  chronological order for the prompt — accumulated context rides along
  with trigger rows up to the cap.
- `MessageInRow` gains `trigger: number` so the container can tell them
  apart in downstream code (container still processes both; only the
  host uses `trigger=0` for don't-wake).

## Defaults (per ACTION-ITEMS item 1 decision)

- DM (is_group=0): `engage_mode='pattern'`, `engage_pattern='.'` (always)
- Threaded group: `engage_mode='mention-sticky'` (seed-discord)
- Non-threaded group / CLI: pattern '.' in bootstrap scripts

## Tests

- src/host-core.test.ts: 3 new cases — fan-out (2 agents, 2 sessions,
  2 wakes), accumulate (trigger=0 + no wake), drop (no session created).
- Existing 10 host-core tests still pass.
- Migration 010 runs on an empty DB in 0-row path — verified.

Closes: ACTION-ITEMS items 1, 4.

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

src/db/migrations/index.ts

@@ -6,6 +6,7 @@ import { migration002 } from './002-chat-sdk-state.js';
 import { moduleAgentToAgentDestinations } from './module-agent-to-agent-destinations.js';
 import { migration008 } from './008-dropped-messages.js';
 import { migration009 } from './009-drop-pending-credentials.js';
+import { migration010 } from './010-engage-modes.js';
 import { moduleApprovalsPendingApprovals } from './module-approvals-pending-approvals.js';
 import { moduleApprovalsTitleOptions } from './module-approvals-title-options.js';
 
@@ -23,6 +24,7 @@ const migrations: Migration[] = [
   moduleApprovalsTitleOptions,
   migration008,
   migration009,
+  migration010,
 ];
 
 export function runMigrations(db: Database.Database): void {
@@ -52,8 +54,8 @@ export function runMigrations(db: Database.Database): void {
   for (const m of pending) {
     db.transaction(() => {
       m.up(db);
-      const next =
-        (db.prepare('SELECT COALESCE(MAX(version), 0) + 1 AS v FROM schema_version').get() as { v: number }).v;
+      const next = (db.prepare('SELECT COALESCE(MAX(version), 0) + 1 AS v FROM schema_version').get() as { v: number })
+        .v;
       db.prepare('INSERT INTO schema_version (version, name, applied) VALUES (?, ?, ?)').run(
         next,
         m.name,