New commands:
- studio cli (alias: c) - Start a new interactive g3 session in an isolated worktree
- studio resume <id> (alias: r) - Resume a paused interactive session
- Bare 'studio' now defaults to 'studio cli'
Session changes:
- Added SessionStatus::Paused for sessions that can be resumed
- Added SessionType enum (OneShot, Interactive) for future use
- Interactive sessions use inherited stdio for direct TTY access
- Sessions are marked as Paused when user exits g3
Workflow:
1. studio # creates worktree, runs g3 interactively
2. (work in g3, exit when done)
3. studio resume <id> # continue working
4. studio accept <id> # merge to main when finished
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