feat: support fast worktree-backed session flows

Add a directory-created fast path for worktree creation so session and send flows can continue once the target directory exists while Git attachment and bootstrap finish in the background.

Track bootstrap status explicitly in shared UI contracts, including pending, ready, and failed states. Background watchers now surface failures and timeouts, update stored worktree metadata, and keep web and VS Code runtime behavior in parity.

Move GitHub issue/PR worktree sessions and assistant-answer fork sessions onto the unified send path so provider, model, agent, and variant selections are preserved. The assistant-answer fork dialog can optionally create a worktree outside VS Code.

Make worktree deletion dialogs close after linked-session cleanup while removing the worktree in the background, and clean up failed fast-create artifacts safely without recursively deleting user or agent-written files.

Validation: bun test packages/ui/src/lib/worktrees/worktreeBootstrap.test.ts packages/ui/src/lib/worktrees/worktreeManager.test.ts; bun run type-check; bun run lint.

packages/ui/src/lib/i18n/messages/ko.ts

@@ -766,6 +766,9 @@ export const dict: Record<I18nKey, string> = {
   'gitView.empty.worktreeFeaturesUnavailable': '이 워크스페이스 모드에서는 워크트리 기능을 사용할 수 없습니다.',
   'gitView.empty.worktreeSetupDescription': '워크트리 설정을 마치고 레포지토리 상태를 준비하고 있습니다.',
   'gitView.empty.worktreeSetupInProgress': '워크트리 설정 중',
+  'worktree.bootstrap.toast.failed': '워크트리 설정 실패',
+  'worktree.bootstrap.toast.failedDescription': '워크트리는 생성되었지만 백그라운드 설정이 완료되지 않았습니다.',
+  'worktree.bootstrap.toast.timeoutDescription': '워크트리는 생성되었지만 백그라운드 설정 시간이 초과되었습니다.',
   'gitView.gitmoji.empty': 'gitmoji 없음',
   'gitView.gitmoji.searchPlaceholder': 'gitmoji 검색…',
   'gitView.gitmoji.title': 'gitmoji 삽입',
@@ -1716,6 +1719,7 @@ export const dict: Record<I18nKey, string> = {
   'chat.messageBody.actions.startNewMultiRun': '이 응답에서 새 멀티런 시작',
   'chat.messageBody.forkDialog.instructions.label': '지침',
   'chat.messageBody.forkDialog.instructions.placeholder': '새 세션에 대한 지침을 입력하세요…',
+  'chat.messageBody.forkDialog.createWorktree': '워크트리 만들기',
   'chat.generatedResult.actions.copy': '복사',
   'chat.generatedResult.actions.copied': '복사됨',
   'chat.generatedResult.commit.title': '생성된 커밋 메시지',