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/src/core/settings-manager.ts

@@ -58,6 +58,7 @@ export interface Settings {
 	terminal?: TerminalSettings;
 	images?: ImageSettings;
 	enabledModels?: string[]; // Model patterns for cycling (same format as --models CLI flag)
+	doubleEscapeAction?: "branch" | "tree"; // Action for double-escape with empty editor (default: "tree")
 }
 
 /** Deep merge settings: project/overrides take precedence, nested objects merge recursively */
@@ -410,4 +411,13 @@ export class SettingsManager {
 	getEnabledModels(): string[] | undefined {
 		return this.settings.enabledModels;
 	}
+
+	getDoubleEscapeAction(): "branch" | "tree" {
+		return this.settings.doubleEscapeAction ?? "tree";
+	}
+
+	setDoubleEscapeAction(action: "branch" | "tree"): void {
+		this.globalSettings.doubleEscapeAction = action;
+		this.save();
+	}
 }