feat(channel-meta): expose globalThis.__fabric.getChannelType for narrow gating

Inbound `message.created` already carries `xType` (dm / triage / group /
broadcast / etc.) — record it in a per-channel cache so other plugins
can answer "is this channel a DM?" without poking the Center API.

New module src/channel-meta.ts:
  - in-memory Map<channelId, xType>
  - lazily loaded from ~/.openclaw/fabric-channel-meta.json on first
    access (so first-ever DM after a fresh gateway start still hits
    cache from the previous run)
  - debounced 250ms flush on dirty; force-flush on gateway_stop
  - recordChannelType(channelId, xType): called from inbound
  - getChannelType(channelId): null if unknown — caller MUST treat null
    as "don't know", NOT as "assume DM" (would re-introduce the false-
    positive on group channels we're trying to eliminate)

Wiring:
  - inbound.ts socket.on('message.created'): records xType BEFORE the
    self-author / dedup gates (channel type is observer-agnostic)
  - index.ts: installs globalThis.__fabric = { getChannelType } on
    registerFull(); flushes on gateway_stop

Consumer: ClawPrompts' fabric-chat-injector will start gating its prompt
injection on getChannelType(channelId) === 'dm' (companion PR on
ClawPrompts). Removes the phase-1 "any fabric channel" false-positive.

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

src/inbound.ts

@@ -6,6 +6,7 @@ import { dispatchInboundReplyWithBase } from 'openclaw/plugin-sdk/inbound-reply-
 import type { FabricClient, FabricSession } from './fabric-client.js';
 import type { IdentityRegistry } from './identity.js';
 import { resolveCoalesce } from './accounts.js';
+import { recordChannelType } from './channel-meta.js';
 import { enqueueDelivery, flushFabricForChannel } from './coalesce.js';
 
 // COMPAT NOTE (openclaw v2026.5.7): the inbound path mirrors how bundled
@@ -401,6 +402,13 @@ export class FabricInbound {
       socket.on('message.created', (m: FabricMessage) => {
         const channelId = m.channelId ?? '';
         if (!channelId) return;
+        // Record xType into the channel-meta cache before self-author
+        // / dedup gates — channel type doesn't depend on who sent the
+        // message, and recording it on observer-only triage messages
+        // is still useful (the next consumer asking
+        // __fabric.getChannelType wants the answer regardless of
+        // whether THIS message was delivered to an agent).
+        recordChannelType(channelId, m.xType);
         if (m.authorUserId && m.authorUserId === selfUserId) return;
         const key = `${agentId}:${m.messageId}`;
         if (this.seen.has(key)) return;