feat: add external terminal support with cross-platform detection

Add support for opening worktree directories in external terminals
(iTerm2, Warp, Ghostty, System Terminal, etc.) while retaining the
integrated terminal as the default option.

Changes:
- Add terminal detection for macOS, Windows, and Linux
- Add "Open in Terminal" split-button in worktree dropdown
- Add external terminal selection in Settings > Terminal
- Add default open mode setting (new tab vs split)
- Display branch name in terminal panel header
- Support 20+ terminals across platforms

Part of #558, Closes #550

libs/types/src/index.ts

@@ -296,3 +296,6 @@ export { EVENT_HISTORY_VERSION, DEFAULT_EVENT_HISTORY_INDEX } from './event-hist
 // Worktree and PR types
 export type { PRState, WorktreePRInfo } from './worktree.js';
 export { PR_STATES, validatePRState } from './worktree.js';
+
+// Terminal types
+export type { TerminalInfo } from './terminal.js';

libs/types/src/settings.ts

@@ -607,6 +607,10 @@ export interface GlobalSettings {
   /** Default editor command for "Open In" action (null = auto-detect: Cursor > VS Code > first available) */
   defaultEditorCommand: string | null;
 
+  // Terminal Configuration
+  /** Default external terminal ID for "Open In Terminal" action (null = integrated terminal) */
+  defaultTerminalId: string | null;
+
   // Prompt Customization
   /** Custom prompts for Auto Mode, Agent Runner, Backlog Planning, and Enhancements */
   promptCustomization?: PromptCustomization;
@@ -896,6 +900,7 @@ export const DEFAULT_GLOBAL_SETTINGS: GlobalSettings = {
   codexThreadId: undefined,
   mcpServers: [],
   defaultEditorCommand: null,
+  defaultTerminalId: null,
   enableSkills: true,
   skillsSources: ['user', 'project'],
   enableSubagents: true,

libs/types/src/terminal.ts

@@ -0,0 +1,15 @@
+/**
+ * Terminal types for the "Open In Terminal" functionality
+ */
+
+/**
+ * Information about an available external terminal
+ */
+export interface TerminalInfo {
+  /** Unique identifier for the terminal (e.g., 'iterm2', 'warp') */
+  id: string;
+  /** Display name of the terminal (e.g., "iTerm2", "Warp") */
+  name: string;
+  /** CLI command or open command to launch the terminal */
+  command: string;
+}