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>

dist/fabric/src/tools.js

@@ -311,10 +311,12 @@ export function registerFabricTools(api, client, identity) {
     api.registerTool((ctx) => ({
         name: 'fabric-guild-list',
         description: 'List guilds the calling agent is a member of. Returns ' +
-            '{nodeId, name, purpose, status} per row. `purpose` is a free-form ' +
-            "description of what each guild is for. Use this BEFORE " +
-            'fabric-channel-list when a workflow asks you to pick the ' +
-            'right guild by intent (no guild ids hardcoded into workflows).',
+            '{nodeId, name, purpose, status} per row. ' +
+            "`purpose` is a free-form description of what each guild is for — " +
+            'pick the guild whose purpose matches your intent. Use this tool ' +
+            'BEFORE fabric-channel-list when a workflow asks you to pick the ' +
+            'right guild by intent (e.g. "find a guild whose purpose mentions ' +
+            'debate broadcasts" → then list its announce-type channels).',
         parameters: {
             type: 'object',
             additionalProperties: false,