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/utils/fs.ts

@@ -0,0 +1,18 @@
+import fs from "node:fs";
+import path from "node:path";
+
+export function ensureDirForFile(filePath: string): void {
+  const dir = path.dirname(filePath);
+  fs.mkdirSync(dir, { recursive: true });
+}
+
+export function readJsonFile<T>(filePath: string, fallback: T): T {
+  if (!fs.existsSync(filePath)) return fallback;
+  const raw = fs.readFileSync(filePath, "utf8");
+  return JSON.parse(raw) as T;
+}
+
+export function writeJsonFile(filePath: string, data: unknown): void {
+  ensureDirForFile(filePath);
+  fs.writeFileSync(filePath, JSON.stringify(data, null, 2), "utf8");
+}