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/cross-plugin-api.ts

@@ -0,0 +1,83 @@
+/**
+ * Cross-plugin API: globalThis.__prismFacet
+ *
+ * PrismFacet is the registration mechanism (routers, rules, the
+ * before_prompt_build hook that resolves them); other plugins (notably
+ * ClawPrompts) supply the actual content by calling into this API at
+ * their own register() time.
+ *
+ * Installation idempotence: safe to call install() multiple times — the
+ * function attaches the same set of methods to globalThis.__prismFacet
+ * if not already present. Plugin reload doesn't break consumers.
+ *
+ * If a consumer plugin (ClawPrompts) loads BEFORE PrismFacet, the
+ * `globalThis.__prismFacet` slot will be undefined at the consumer's
+ * register() time. To make load-order independence work, this module
+ * pre-creates the slot at module-import time too — consumers can call
+ * .addRouter / .addRule even if PrismFacet's register() hasn't run yet
+ * (the registration still lands in the shared maps).
+ */
+import { addExternalRouter, type RouterContext } from "./router-loader.js";
+import { addExternalRule } from "./rule-store.js";
+
+const _G = globalThis as Record<string, unknown>;
+
+export interface PrismFacetCrossPluginApi {
+  /**
+   * Register a router programmatically.
+   * @param name router name (matches the "router:" prefix of rule keys)
+   * @param resolve function returning a key string for the given context
+   *                (or empty/null to skip this router for this context)
+   */
+  addRouter(
+    name: string,
+    resolve: (ctx: RouterContext) => string | Promise<string>,
+  ): void;
+
+  /**
+   * Register a rule programmatically (in-memory; persistent rules.json
+   * entries set by the prompt-rules tool take precedence on conflict).
+   * @param routerName router this rule binds to
+   * @param key value the router must resolve to for this rule to fire
+   * @param prompt either { file: "/abs/path.md" } or { text: "..." }
+   */
+  addRule(
+    routerName: string,
+    key: string,
+    prompt: { file?: string; text?: string },
+  ): void;
+}
+
+/** Idempotent install. Called from PrismFacet's register(). Also runs
+ * at module-import time below so that consumer plugins loading before
+ * PrismFacet still find a usable API. */
+export function installCrossPluginApi(): PrismFacetCrossPluginApi {
+  const existing = _G["__prismFacet"] as PrismFacetCrossPluginApi | undefined;
+  if (existing && typeof existing.addRouter === "function") return existing;
+
+  const api: PrismFacetCrossPluginApi = {
+    addRouter(name, resolve) {
+      addExternalRouter(name, resolve);
+    },
+    addRule(routerName, key, prompt) {
+      // Phase 1: only file-based prompts are wired through the existing
+      // injector. `text` inline prompts will need a tmpfile-or-store
+      // backend; surfaced as a TODO so an early caller failing loudly
+      // beats silently dropping the prompt.
+      if (!prompt.file) {
+        throw new Error(
+          "__prismFacet.addRule: only { file: '/abs/path.md' } is supported today; " +
+          "inline { text } pending injector update",
+        );
+      }
+      addExternalRule(routerName, key, prompt.file);
+    },
+  };
+  _G["__prismFacet"] = api;
+  return api;
+}
+
+// Eager install on module load so a consumer plugin importing into the
+// same gateway process can register even if it runs before PrismFacet's
+// register() does.
+installCrossPluginApi();