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/docs/extensions.md

@@ -946,6 +946,17 @@ pi.registerTool({
 
 Extensions can override built-in tools (`read`, `bash`, `edit`, `write`, `grep`, `find`, `ls`) by registering a tool with the same name. Interactive mode displays a warning when this happens.
 
+Use `--no-tools` to start without built-in tools, then add back specific ones with `--tools`:
+```bash
+# No built-in tools, only extension tools
+pi --no-tools -e ./my-extension.ts
+
+# No built-in read, use extension's read, keep other built-ins
+pi -e ./tool-override.ts
+```
+
+See [examples/extensions/tool-override.ts](../examples/extensions/tool-override.ts) for a complete example that overrides `read` with logging and access control.
+
 **Your implementation must match the exact result shape**, including the `details` type. The UI and session logic depend on these shapes for rendering and state tracking.
 
 Built-in tool implementations: