feat(coding-agent): configurable double-escape action (tree vs branch)

Add doubleEscapeAction setting to choose whether double-escape with an
empty editor opens /tree (default) or /branch.

- Add setting to Settings interface and SettingsManager
- Add to /settings UI for easy toggling
- Update interactive-mode to respect the setting
- Document in README.md settings table

fixes #404

packages/coding-agent/README.md

@@ -586,6 +586,7 @@ Global `~/.pi/agent/settings.json` stores persistent preferences:
 | `retry.baseDelayMs` | Base delay for exponential backoff | `2000` |
 | `terminal.showImages` | Render images inline (supported terminals) | `true` |
 | `images.autoResize` | Auto-resize images to 2000x2000 max for better model compatibility | `true` |
+| `doubleEscapeAction` | Action for double-escape with empty editor: `tree` or `branch` | `tree` |
 | `hooks` | Additional hook file paths | `[]` |
 | `customTools` | Additional custom tool file paths | `[]` |