feat(cloud-agents): add no-repo mode, fix session init race, enable follow-ups after cloud run

[tatoalo]

Problem

Three issues with cloud task follow-ups in the desktop app:

1. Race condition: SSE connection and autoInitializeSession() can both call
   initializeSession() concurrently, creating duplicate sessions and sending the
   initial prompt twice (duplicate Slack messages).
2. Follow-up replies not delivered: after a follow-up prompt completes, the
   agent response was not being relayed back to Slack.
3. Chat input disabled: PR feat: Add multi-agent command center grid view #1212 re-introduced readOnlyMessage and changed
   isRunning to be false when cloud run is terminal, blocking the editor after
   task completion.

Changes

• Add initializationPromise lock to prevent concurrent session creation, with
  pendingEvents buffer that replays SSE events after the session attaches.
• After end_turn on follow-ups and resumes, call relayAgentResponse() to post
  the agent's reply to Slack. Return assistant_message in the prompt response for
  callers that need it synchronously.
• Add currentTurnMessages tracking in SessionLogWriter with
  getFullAgentResponse() and resetTurnMessages() for accurate per-turn message
  extraction.
• Add "No Repository Mode" system prompt for cloud tasks without a connected repo.
• Remove readOnlyMessage from useSessionViewState, SessionView,
  TaskLogsPanel, and CommandCenterSessionView. Restore isRunning = true for
  cloud tasks so the editor stays enabled after completion.
• Improve sandbox restore messaging to be more user-friendly.