chore: convert plugin to ESM and migrate to current openclaw plugin SDK

ESM conversion:
- plugin/package.json: add "type": "module".
- plugin/tsconfig.json: switch module/moduleResolution to "nodenext"; bump
  target to ES2022.
- All relative imports across plugin/ now carry .js extensions as required
  by Node ESM (nodenext module resolution).
- plugin/index.ts: replace `require('./calendar/schedule-cache')` with a
  proper top-level import; switch `from 'os'` to `from 'node:os'`.

Plugin SDK convention update:
- Wrap default export with definePluginEntry({ id, name, description,
  register }) per the current openclaw plugin authoring contract.
- Modernize plugin/openclaw.plugin.json: drop entry/version, add
  activation.onStartup so gateway_start fires for this plugin at boot,
  declare contracts.tools listing the five harborforge_* tools.
- Add a local plugin/openclaw-sdk.d.ts with ambient declarations for the
  focused subpaths (openclaw/plugin-sdk/plugin-entry,
  openclaw/plugin-sdk/core). We deliberately do NOT add openclaw as an
  npm devDependency: the installer's `npm install --omit=dev` step trips
  over openclaw's own (deeply nested) dependency graph when listed via
  file:.../openclaw, and the runtime contract is provided by the gateway
  loader anyway.
- The local PluginAPI interface is preserved (broader than the standard
  OpenClawPluginApi — it surfaces `version`/`runtime`/`spawn`); the
  register function is cast at the definePluginEntry boundary.

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

plugin/tsconfig.json

@@ -1,8 +1,8 @@
 {
   "compilerOptions": {
     "target": "ES2022",
-    "module": "CommonJS",
-    "moduleResolution": "node",
+    "module": "nodenext",
+    "moduleResolution": "nodenext",
     "esModuleInterop": true,
     "strict": true,
     "skipLibCheck": true,