docs(changelog): document breaking changes in [Unreleased]

Add comprehensive documentation for three breaking changes:
1. Runtime auto-migration from v1 to v2 config format removed
2. Legacy v1 copy file migration logic removed
3. Tauri commands now only accept 'app' parameter

Also document improvements and new tests added in recent commits.

CHANGELOG.md

@@ -257,13 +257,35 @@ For users upgrading from v2.x (Electron version):
 
 ### ⚠️ Breaking Changes
 
-- Tauri 命令统一仅接受 `app` 参数，移除历史 `app_type`/`appType` 兼容路径；传入未知 `app` 时会明确报错，并提示可选值。
+- **Runtime auto-migration from v1 to v2 config format has been removed**
+  - `MultiAppConfig::load()` no longer automatically migrates v1 configs
+  - When a v1 config is detected, the app now returns a clear error with migration instructions
+  - **Migration path**: Install v3.2.x to perform one-time auto-migration, OR manually edit `~/.cc-switch/config.json` to v2 format
+  - **Rationale**: Separates concerns (load() should be read-only), fail-fast principle, simplifies maintenance
+  - Related: `app_config.rs` (v1 detection improved with structural analysis), `app_config_load.rs` (comprehensive test coverage added)
+
+- **Legacy v1 copy file migration logic has been removed**
+  - Removed entire `migration.rs` module (435 lines) that handled one-time migration from v3.1.0 to v3.2.0
+  - No longer scans/merges legacy copy files (`settings-*.json`, `auth-*.json`, `config-*.toml`)
+  - No longer archives copy files or performs automatic deduplication
+  - **Migration path**: Users upgrading from v3.1.0 must first upgrade to v3.2.x to automatically migrate their configurations
+  - **Benefits**: Improved startup performance (no file scanning), reduced code complexity, cleaner codebase
+
+- **Tauri commands now only accept `app` parameter**
+  - Removed legacy `app_type`/`appType` compatibility paths
+  - Explicit error with available values when unknown `app` is provided
 
 ### 🔧 Improvements
 
-- 统一 `AppType` 解析：集中到 `FromStr` 实现，命令层不再各自实现 `parse_app()`，减少重复与漂移。
-- 错误消息本地化与更友好：对不支持的 `app` 返回中英双语提示，并包含可选值清单。
+- Unified `AppType` parsing: centralized to `FromStr` implementation, command layer no longer implements separate `parse_app()`, reducing code duplication and drift
+- Localized and user-friendly error messages: returns bilingual (Chinese/English) hints for unsupported `app` values with a list of available options
+- Simplified startup logic: Only ensures config structure exists, no migration overhead
 
 ### 🧪 Tests
 
-- 新增单元测试覆盖 `AppType::from_str`：大小写、裁剪空白、未知值错误消息。
+- Added unit tests covering `AppType::from_str`: case sensitivity, whitespace trimming, unknown value error messages
+- Added comprehensive config loading tests:
+  - `load_v1_config_returns_error_and_does_not_write`
+  - `load_v1_with_extra_version_still_treated_as_v1`
+  - `load_invalid_json_returns_parse_error_and_does_not_write`
+  - `load_valid_v2_config_succeeds`