fix(channels): pre-subscribe group threads for pattern / accumulate wirings

The engage modes shipped in #1869 included `pattern` (regex match any
message) and the `accumulate` ignored-message policy, but neither could
fire in group chats because Chat SDK only surfaces:

  - DMs (onDirectMessage)
  - @mentions in unsubscribed threads (onNewMention)
  - every message in subscribed threads (onSubscribedMessage)

A bot sitting in a Discord/Slack channel hears *nothing* from a plain
message unless the thread is already subscribed. So `pattern '.'` on a
group wiring → silent. `pattern /urgent/i` → silent. `mention +
accumulate` → the non-mention messages that should be stored as context
were never received, so nothing to accumulate.

Fix: call `chat.subscribe(platformId)` at setup time for every wiring
whose `engageMode === 'pattern'` or `ignoredMessagePolicy === 'accumulate'`.
Failures logged + swallowed per-conversation so one un-subscribable
channel doesn't crash startup.

## Knock-on: SDK stops firing onNewMention once subscribed

Per SDK types:1468, `onNewMention` only fires in unsubscribed threads.
Once we pre-subscribe a channel for a pattern wiring, subsequent mentions
arrive as `onSubscribedMessage` with `message.isMention === true`.

Before: a `mention` wiring coexisting with a `pattern` wiring in the
same channel would silently stop firing after pre-subscribe.

After: `shouldEngage` accepts the `isMention` flag independently from
`source`, so the `mention` mode matches on (dm OR mention-new OR
subscribed-with-isMention). Source shape changed
`'subscribed' | 'mention' | 'dm'` → `'subscribed' | 'mention-new' | 'dm'`
to make the "unsubscribed-mention event" distinction explicit.

## New fields

- `ConversationConfig.ignoredMessagePolicy` — projected from the
  messaging_group_agents row so the bridge knows which wirings need
  pre-subscription. buildConversationConfigs in src/index.ts populates
  it.

Tests: host 153/153, container 46/46. No new tests yet — the subscribe
call path needs a Chat mock, deferred.

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

src/channels/adapter.ts

@@ -22,6 +22,13 @@ export interface ConversationConfig {
   engageMode: 'pattern' | 'mention' | 'mention-sticky';
   /** Regex source when engageMode='pattern'. '.' is the "always" sentinel. */
   engagePattern?: string | null;
+  /**
+   * What to do with non-engaging messages. Projected from the wiring so the
+   * adapter can decide whether to pre-subscribe to group threads — `accumulate`
+   * means "store everything as context even when not engaging", which requires
+   * seeing every message in the thread.
+   */
+  ignoredMessagePolicy?: 'drop' | 'accumulate';
   sessionMode: 'shared' | 'per-thread' | 'agent-shared';
 }