feat: get_data_dir() three-tier fallback (env -> ~/.cmbot -> dev root)

Packaged builds now resolve the data root to ~/.cmbot (%USERPROFILE%\.cmbot)
instead of the program dir, so user config/templates/output stay writable and
per-user regardless of where the program is installed or whether it is launched
via Launcher.exe (docs/10-lan-update.md §5). CMBOT_DATA_DIR still overrides.

- tests/test_file_service.py: 5 tests covering env override, frozen->~/.cmbot,
  dev->project root, blank-env handling

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tasks.md

@@ -932,7 +932,7 @@
 任务：
 
 - [x] 文档：`docs/10` 改为便携 + `Launcher.exe` + `~/.cmbot` 模型（§3/§4/§5/§8/§9/§11/§16）
-- [ ] `get_data_dir()` 三级回退：`CMBOT_DATA_DIR` → 打包态 `~/.cmbot` → 开发态项目根；更新单测
+- [x] `get_data_dir()` 三级回退：`CMBOT_DATA_DIR` → 打包态 `~/.cmbot` → 开发态项目根；`tests/test_file_service.py` 5 个单测
 - [ ] `src/launcher.py`：复用 `update_service`，下载 zip→SHA-256→解压→`app/app.old` 切换→启动；安装根可写性检测；首次把 `app\config\` 默认模板播种到 `~/.cmbot`
 - [ ] `build.ps1` 增产 `Launcher.exe`（PyInstaller onefile），发布 zip 含 `Launcher.exe` + `app\`
 - [ ] 退休 `scripts/update.ps1` 与 `scripts/install_local.ps1`