feat(hooks): fabric-chat-injector — suggest chat workflow on channel turns

New before_prompt_build hook that appends a "next action: workflow_start
chat" hint to the system prompt whenever the agent's turn was triggered
by a message in a fabric channel.

If Meridian (`globalThis.__meridian.getChatJournalForChannel`) reports
an existing chat journal for this (agentId, channelId), the hint
includes `from="<journal-id>"` so the agent resumes the conversation
file instead of starting a fresh one each turn.

Activation:
  - ctx.agentId AND ctx.channelId present
  - ctx.messageProvider in {fabric, '' (empty/omitted by gateway)}

TODO(phase-2): once Fabric exposes per-channel type info (DM / group /
triage) via a cross-plugin API, narrow this to xType === 'dm' only.
Today we fire on any fabric channel — chat workflow is a no-op outside
DMs, so the false positives are just prompt-text noise.

Dedup via WeakSet keyed on the event object (same pattern as the
existing before-prompt-build hook) so each turn injects at most once
even when multiple harness call sites trigger the hook.

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

plugin/hooks/fabric-chat-injector.ts

@@ -0,0 +1,85 @@
+/**
+ * Inject a "start the chat workflow" hint into the agent's system prompt
+ * when this turn was triggered by a message in a fabric channel.
+ *
+ * Two cases:
+ *   - no prior chat journal for this (agent, channel) → suggest
+ *     `workflow_start chat` (fresh).
+ *   - mapping exists → suggest `workflow_start chat from <journalId>`
+ *     so the conversation continues in the same linear journal.
+ *
+ * The (channel → journal) mapping is owned by Meridian and exposed via
+ * globalThis.__meridian.getChatJournalForChannel. Meridian writes the
+ * entry the first time the agent starts a chat workflow on a channel
+ * with no `from` argument.
+ *
+ * TODO(phase-2): once Fabric.OpenclawPlugin exposes per-channel type
+ * info (DM / group / triage) via a cross-plugin API, narrow this hook
+ * to xType === 'dm' only. Today we inject for any fabric channel — chat
+ * workflow itself is a no-op outside DMs, but the suggestion is noise.
+ */
+const _G = globalThis as Record<string, unknown>;
+const DEDUP_KEY = '_prismFacetFabricChatDedup';
+
+interface MeridianBridge {
+  getChatJournalForChannel?: (agentId: string, channelId: string) => Promise<string | null>;
+}
+
+interface PromptCtx {
+  agentId?: string;
+  channelId?: string;
+  messageProvider?: string;
+}
+
+export function registerFabricChatInjector(api: {
+  on(hook: string, handler: (...args: any[]) => any): void;
+  logger: { info(msg: string): void; warn(msg: string): void };
+}): void {
+  if (!(_G[DEDUP_KEY] instanceof WeakSet)) _G[DEDUP_KEY] = new WeakSet<object>();
+  const dedup = _G[DEDUP_KEY] as WeakSet<object>;
+
+  api.on('before_prompt_build', async (event: unknown, ctx: PromptCtx) => {
+    if (dedup.has(event as object)) return;
+    dedup.add(event as object);
+
+    const agentId = ctx.agentId || '';
+    const channelId = (ctx.channelId || '').trim();
+    const provider = (ctx.messageProvider || '').toLowerCase();
+    if (!agentId || !channelId) return;
+    if (provider && provider !== 'fabric') return;
+    // Empty provider also accepted — gateway sometimes omits the field
+    // even when the trigger was a fabric channel; channelId presence is
+    // the load-bearing signal.
+
+    let journalId: string | null = null;
+    const meridian = _G['__meridian'] as MeridianBridge | undefined;
+    if (typeof meridian?.getChatJournalForChannel === 'function') {
+      try {
+        journalId = await meridian.getChatJournalForChannel(agentId, channelId);
+      } catch (err) {
+        api.logger.warn(
+          `[prism-facet] fabric-chat-injector: meridian lookup failed for ` +
+          `agent=${agentId} channel=${channelId}: ${String(err)}`,
+        );
+      }
+    }
+
+    const cmd = journalId
+      ? `\`workflow_start\` with \`workflow="chat"\` and \`from="${journalId}"\``
+      : `\`workflow_start\` with \`workflow="chat"\``;
+
+    const continuationLine = journalId
+      ? `This channel already has an open chat journal (\`${journalId}\`). Resume it with the \`from\` argument so the conversation history stays in one file.`
+      : `No prior chat journal exists for this channel yet — Meridian will create a fresh one and remember the channel→journal mapping for future turns.`;
+
+    const segment =
+      `# Chat channel context\n` +
+      `\n` +
+      `This turn was triggered by a message in a fabric channel (\`${channelId}\`).\n` +
+      `${continuationLine}\n` +
+      `\n` +
+      `**Next action:** call ${cmd} to enter the chat workflow before doing anything else.\n`;
+
+    return { appendSystemContext: segment };
+  });
+}