Merge hooks and custom-tools into unified extensions system (#454)

Breaking changes:
- Settings: 'hooks' and 'customTools' arrays replaced with 'extensions'
- CLI: '--hook' and '--tool' flags replaced with '--extension' / '-e'
- API: HookMessage renamed to CustomMessage, role 'hookMessage' to 'custom'
- API: FileSlashCommand renamed to PromptTemplate
- API: discoverSlashCommands() renamed to discoverPromptTemplates()
- Directories: commands/ renamed to prompts/ for prompt templates

Migration:
- Session version bumped to 3 (auto-migrates v2 sessions)
- Old 'hookMessage' role entries converted to 'custom'

Structural changes:
- src/core/hooks/ and src/core/custom-tools/ merged into src/core/extensions/
- src/core/slash-commands.ts renamed to src/core/prompt-templates.ts
- examples/hooks/ and examples/custom-tools/ merged into examples/extensions/
- docs/hooks.md and docs/custom-tools.md merged into docs/extensions.md

New test coverage:
- test/extensions-runner.test.ts (10 tests)
- test/extensions-discovery.test.ts (26 tests)
- test/prompt-templates.test.ts

packages/coding-agent/docs/tui.md

@@ -343,4 +343,4 @@ Call `invalidate()` when state changes, then `handle.requestRender()` to trigger
 ## Examples
 
 - **Snake game**: [examples/hooks/snake.ts](../examples/hooks/snake.ts) - Full game with keyboard input, game loop, state persistence
-- **Custom tool rendering**: [examples/custom-tools/todo/](../examples/custom-tools/todo/) - Custom `renderCall` and `renderResult`
+- **Custom tool rendering**: [examples/extensions/todo.ts](../examples/extensions/todo.ts) - Custom `renderCall` and `renderResult`