Flow PR

End-to-end PR lifecycle for a unit of work: default branch detection, branch cutting, commit → push → PR open, review gate, CI gate, and merge. The policy layer on top of existing mechanics.

Defers:

• Commit message policy → /commit
• Code-review logic → /code-review (flow-pr runs it as the review gate; does not implement it)
• Issue filing → repo's intake convention
• Merge mechanics → merge-mechanics.md

/code-review here means the installed two-axis Standards+Spec skill (Matt's), which is what resolves for that name today (#556).

Default branch detection