- Add ensure_worktree() - creates worktree if missing, returns status
- Add ensure_session() - creates session if missing, handles inconsistent states
- Add fork_agent() - extracted agent forking logic
- Refactor cmd_continue() to use ensure_* functions (idempotent)
- Make cmd_start() a thin wrapper calling cmd_continue()
- Simplify daemon to always call cmd_continue (no existence check)
This makes cmd_continue truly idempotent - it will:
- Continue existing session if it exists
- Create session and worktree if they don't exist
- Clean and recreate if state is inconsistent
Closes#168
d240000088