feat: implement hooks, plugins, session forks, and vim motions

Add 4 major features to codetyper-cli:

- Hooks System: Lifecycle hooks (PreToolUse, PostToolUse, SessionStart,
  SessionEnd, UserPromptSubmit, Stop) with exit code control flow
- Plugin System: Custom tools, commands, and hooks via plugin manifest
- Session Forking: Snapshots, rewind, fork, and switch between branches
- Vim Motions: Normal/Insert/Command/Visual modes with keyboard navigation

New files:
- src/types/{hooks,plugin,session-fork,vim}.ts
- src/constants/{hooks,plugin,session-fork,vim}.ts
- src/services/{hooks-service,plugin-loader,plugin-service,session-fork-service}.ts
- src/stores/vim-store.ts (vanilla)
- src/tui/hooks/{useVimMode,useVimStore,useTodoStore,useThemeStore}.ts
- src/tui/components/VimStatusLine.tsx

Modified:
- src/services/agent.ts (hook integration)
- src/tools/index.ts (plugin tool registration)
- src/stores/{todo-store,theme-store}.ts (converted to vanilla)
- TUI components (updated hook imports)

docs/CHANGELOG.md

@@ -9,6 +9,38 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- **Hooks System**: Lifecycle hooks for extensibility
+  - 6 hook events: PreToolUse, PostToolUse, SessionStart, SessionEnd, UserPromptSubmit, Stop
+  - Exit code control flow (0=allow, 1=warn, 2=block)
+  - JSON input/output via stdin/stdout
+  - Modified arguments via `updatedInput`
+  - Global + local configuration support
+  - Configurable timeout per hook
+
+- **Plugin System**: Custom tools, commands, and hooks
+  - Plugin manifest with version and capabilities
+  - Custom tool definitions via TypeScript
+  - Custom slash commands via Markdown with frontmatter
+  - Plugin-specific hooks
+  - Global (~/.config/codetyper/plugins/) + local (.codetyper/plugins/)
+  - Dynamic tool/command registration
+
+- **Session Forking/Rewind**: Branch and time-travel session history
+  - Named snapshots at any point in conversation
+  - Rewind to any snapshot by name or index
+  - Fork branches from any snapshot
+  - Switch between forks
+  - Suggested commit messages based on session content
+  - Commands: /snapshot, /rewind, /fork, /forks, /switch
+
+- **Vim Motions**: Vim-style keyboard navigation
+  - 4 modes: Normal, Insert, Command, Visual
+  - Scroll navigation (j/k, gg/G, Ctrl+d/u)
+  - Search with highlighting (/, n/N)
+  - Command mode (:q, :w, :wq, :nohl)
+  - Mode indicator in status line
+  - Configurable via settings.json
+
 - **Home Screen**: New welcome screen with centered gradient logo
   - Displays version, provider, and model info
   - Transitions to session view on first message