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/package.json

@@ -0,0 +1,6 @@
+{
+  "name": "prism-facet",
+  "version": "0.1.0",
+  "type": "module",
+  "main": "index.js"
+}