refactor: restructure PrismFacet per OpenClaw plugin spec

- plugin/ directory structure: hooks/, tools/, core/
- export default { id, name, register } entry format
- globalThis state management with lifecycle protection
- WeakSet dedup on before_prompt_build hook
- Tool uses inputSchema + execute (not parameters + handler)
- additionalProperties: false in config schema
- Core logic in plugin/core/ (no plugin-sdk dependency)
- Install/uninstall script (scripts/install.mjs)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

plugin/hooks/before-prompt-build.ts

@@ -0,0 +1,25 @@
+import { resolveInjection } from "../core/prompt-injector.js";
+
+const _G = globalThis as Record<string, unknown>;
+const DEDUP_KEY = "_prismFacetBPBDedup";
+
+export function registerBeforePromptBuild(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: { agentId?: string }) => {
+    if (dedup.has(event as object)) return;
+    dedup.add(event as object);
+
+    const agentId = ctx.agentId || "";
+    if (!agentId) return;
+
+    const result = await resolveInjection({ agentId }, api.logger);
+    if (result.appendSystemContext) {
+      return { appendSystemContext: result.appendSystemContext };
+    }
+  });
+}