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

.pi/settings.json

@@ -1,4 +0,0 @@
-{
-  "customTools": ["packages/coding-agent/examples/custom-tools/todo/index.ts"],
-  "hooks": ["packages/coding-agent/examples/hooks/todo/index.ts"]
-}