refactor: restructure project layout and add install.mjs

- Move src/ → plugin/ with subdirectories:
  - plugin/core/ (business logic, models, store, permissions, utils, memory)
  - plugin/tools/ (query, resources)
  - plugin/commands/ (placeholder for slash commands)
  - plugin/hooks/ (placeholder for lifecycle hooks)
  - plugin/index.ts (wiring layer only, no business logic)
- Add install.mjs with --install, --uninstall, --openclaw-profile-path
- Add skills/ and docs/ root directories
- Move planning docs (PLAN.md, FEAT.md, AGENT_TASKS.md) to docs/
- Remove old scripts/install.sh
- Update tsconfig rootDir: src → plugin
- Update README.md and README.zh.md with new layout
- Bump version to 0.2.0
- All tests pass

scripts/demo.ts

@@ -1,6 +1,6 @@
 import path from 'node:path';
 import fs from 'node:fs';
-import { Yonexus } from '../src/index';
+import { Yonexus } from '../plugin/index';
 
 const dataFile = path.resolve(process.cwd(), 'data/demo-org.json');
 if (fs.existsSync(dataFile)) fs.unlinkSync(dataFile);