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

plugin/core/models/errors.ts

@@ -0,0 +1,19 @@
+export type ErrorCode =
+  | 'PERMISSION_DENIED'
+  | 'NOT_FOUND'
+  | 'ALREADY_EXISTS'
+  | 'VALIDATION_ERROR'
+  | 'FIELD_NOT_QUERYABLE'
+  | 'INVALID_SUPERVISOR'
+  | 'REGISTRAR_DENIED';
+
+export class YonexusError extends Error {
+  constructor(
+    public readonly code: ErrorCode,
+    message: string,
+    public readonly details?: Record<string, unknown>
+  ) {
+    super(message);
+    this.name = 'YonexusError';
+  }
+}