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/core/router-loader.ts

@@ -75,3 +75,21 @@ export function getRouters(): LoadedRouter[] {
 export function getRouterNames(): string[] {
   return Array.from(getRouterMap().keys());
 }
+
+/**
+ * Cross-plugin API: register a router programmatically. Other plugins
+ * (e.g. ClawPrompts) call this via globalThis.__prismFacet.addRouter
+ * to publish a router without dropping a .ts file in PrismFacet's
+ * routersDir. Replaces any existing router of the same name.
+ */
+export function addExternalRouter(
+  name: string,
+  resolveFn: (ctx: RouterContext) => string | Promise<string>,
+): void {
+  const map = getRouterMap();
+  map.set(name, {
+    name,
+    filePath: `<external: registered via __prismFacet.addRouter>`,
+    module: { resolve: resolveFn },
+  });
+}