Task chaining: --after flag for sequential dependencies

[chanakyav]

Support sequential task dependencies where one task waits for another to complete before starting.

Use Case

Common workflow: PR 1 -> PR 2 (based on PR 1) -> PR 3 (based on PR 2). You want to queue all three upfront and have them execute in sequence, each branching off the previous one's branch.

Proposed Design

Option A: --after flag

# Start first task
autopilot start --prompt "Add user model" 
# => task abc123

# Queue second task — starts automatically when abc123 completes
autopilot start --prompt "Add user API endpoints" --after abc123
# => task def456 (state: WAITING)

# Queue third task
autopilot start --prompt "Add user UI" --after def456

Option B: --base flag (branch off another autopilot branch)

autopilot start --prompt "Add endpoints" --base autopilot/abc123

This starts immediately but works on a branch based on the parent branch rather than main.

Behavior

• --after <task_id> creates a task in WAITING state
• Orchestrator polls until the dependency reaches COMPLETE
• If dependency reaches FAILED or STOPPED, the waiting task also fails with a clear message
• Child task branches off the parent's branch (not main)
• After parent PR merges, child task should rebase onto main

Implementation Notes

• New WAITING state in the state machine
• New DB columns: depends_on_task_id, base_branch
• Background polling or event-driven trigger when parent completes
• Consider: what if parent's PR hasn't merged yet? Child should branch off parent's branch
• Requires git worktrees (#TBD) for parallel execution during the transition period

Open Questions

• Should --after auto-detect the parent's branch, or require --base explicitly?
• What happens if the parent task is stopped? Should children be stopped too?
• Should we support fan-out (multiple tasks depending on same parent)?