feat: Bake Git discipline into plan execution at every level

[mmorris35]

Generated plans are putting Git strategy at top/bottom, but Haiku agent needs Git commands baked into every level for proper execution.

Proposed Strategy:

• Phase level: Create new feature branch OR worktree at start of each phase
• Task level: Push at completion of each task, merge to main when task completes
• Subtask level: Commit and push at completion of each subtask

Questions:

1. Work trees vs feature branches - which works better for Haiku agent execution?
2. Work tree/branch at phase level or task level?
3. Is commit+push at subtask level correct?

Current problem: Plans have Git strategy in header/footer but it's not executable by Haiku - needs to be inline at each level.