refactor(prism-facet): become a pure registration framework

Strip all Hangman-Lab-specific content out of PrismFacet so it can be
reused by any project. Content (always router, pcexec-convention prompt,
fabric-chat-injector hook) moves to the new sibling plugin ClawPrompts.

Mechanism additions:
- `globalThis.__prismFacet` cross-plugin API installed at module-import
  time (so consumers loaded before PrismFacet can still register):
    .addRouter(name, resolveFn)
    .addRule(router, key, { file })
- core/rule-store: tier rules into `persistent` (rules.json, mutated by
  the prompt-rules admin tool) and `external` (in-memory, registered by
  other plugins via the API). Persistent overrides external on conflict.
- core/router-loader: addExternalRouter() for programmatic registration
  into the same map the file-based loader uses.
- index.ts: drops registerFabricChatInjector wiring, registerBeforePromptBuild
  remains.

Removed (now shipped from ClawPrompts):
- plugin/routers/always.ts
- plugin/hooks/fabric-chat-injector.ts
- plugin/prompts/pcexec-convention.md
- plugin/rules.json: now `{}`; ClawPrompts registers its rule externally

What still lives in PrismFacet:
- before_prompt_build hook (the wiring between routers/rules and the
  agent's system prompt)
- prompt-rules admin tool (lists + mutates persistent rules)
- file-based routersDir / rulesFile scanning (kept for operator ad-hoc
  use; ClawPrompts uses the API instead)

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

plugin/hooks/fabric-chat-injector.ts

@@ -1,85 +0,0 @@
-/**
- * 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 };
-  });
-}