Custom process: Self-merge Step and tighten-only limits #1390

Parent

#877 (Spec: Custom processes). Design record: docs/grill-with-docs/877-composable-by-file.md, ADR-0024.

What to build

The self-merge catalogue Step, directly after delivery, is the existing Self-merge. It merges only when the command's merge flag or the User config's merge_always asks for one. Otherwise the pull request stays ready for review. A file can only tighten limits, never loosen them (ADR-0024):

• repair_cap no higher than the effective cap;
• self_merge = false to forbid a merge;
• protected_paths (globs): a change touching one holds the Self-merge and leaves the pull request ready for review, as an unaddressed Security finding holds one.

Acceptance criteria

• With the merge flag, a Process ending in delivery and then self-merge merges as a Ticket run asked to merge does. Without it, the pull request is left ready for review.
• self-merge anywhere but directly after delivery is refused.
• A repair_cap above the effective cap is refused as a loosening limit. A lower one stops the Repair loop at that cap.
• self_merge = false holds the merge even with the merge flag, and the outcome says why.
• A change to a protected path holds the merge, leaves the pull request ready for review, and names the path.
• docs/guide.md documents the limits and that they only tighten.