feat(coding-agent): add tool override support via extensions

- Add setActiveTools() to ExtensionAPI for dynamic tool management
- Extensions can now override, wrap, or disable built-in tools
- Add tool-override.ts example demonstrating the pattern
- Update documentation for tool override capabilities

packages/coding-agent/examples/extensions/README.md

@@ -30,6 +30,7 @@ cp permission-gate.ts ~/.pi/agent/extensions/
 | `todo.ts` | Todo list tool + `/todos` command with custom rendering and state persistence |
 | `hello.ts` | Minimal custom tool example |
 | `question.ts` | Demonstrates `ctx.ui.select()` for asking the user questions |
+| `tool-override.ts` | Override built-in tools (e.g., add logging/access control to `read`) |
 | `subagent/` | Delegate tasks to specialized subagents with isolated context windows |
 
 ### Commands & UI