feat: /pre-merge skill for MR/PR review before submission

[backspace-shmackspace]

Context

When preparing a merge request, there's a gap between the /ship verification (which runs at
implementation time) and the actual MR submission. Code changes accumulate after /ship — bug fixes,
CI config changes, rebase conflict resolutions — that never go through code review or security
scan.

Proposal

A /pre-merge skill that automatically:

1. Diffs the current branch against the target branch (origin/main)
2. Runs a scoped /code-review on the diff (not the full codebase)
3. Runs a scoped /secure-review on the diff (if deployed)
4. Validates CI config files (YAML syntax, no conflict markers)
5. Checks for common MR hygiene issues (uncommitted changes, secrets in diff, large binary files)
6. Produces a summary: READY / NEEDS_ATTENTION with findings

Lighter than /audit, heavier than manual review. Designed to run in 2-5 minutes as a pre-submission
gate.

Motivation

During a real session , accumulated 9 commits across /ship, /fix, CI config changes, and rebase. The final MR had unresolved merge conflict markers inherited during rebase, a thread-safety bug in a CVE patch, and a shell variable scoping issue in CI config. All caught by manually running /code-review at the end — a /pre-merge skill would have caught them automatically.

Implementation

New skill in skills/pre-merge/SKILL.md following the coordinator pattern. Composes /code-review and
/secure-review as subagents, similar to how /audit composes them.