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/calendar/calendar-bridge.ts

@@ -29,7 +29,7 @@ import {
   CalendarSlotResponse,
   SlotAgentUpdate,
   SlotStatus,
-} from './types';
+} from './types.js';
 
 export interface CalendarBridgeConfig {
   /** HarborForge backend base URL (e.g. "https://monitor.hangman-lab.top") */
@@ -292,7 +292,7 @@ export class CalendarBridgeClient {
 // ---------------------------------------------------------------------------
 
 import { hostname } from 'os';
-import { getPluginConfig } from '../core/config';
+import { getPluginConfig } from '../core/config.js';
 
 export interface CalendarPluginConfig {
   /** Backend URL for calendar API (overrides monitor backendUrl) */