feat(merge-fix): find a fix across branches and merge it

Fetches every ref, searches commit messages, content and paths for the fix described in the skill argument, drops what HEAD already contains, and brings the survivor over via cherry-pick or a no-ff merge. Dirty trees are refused up front so no stash or checkout can eat another agent's uncommitted work in the shared tree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

skills/merge-fix/SKILL.md

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+---
+name: merge-fix
+description: >
+  Fetch every branch of the repository, hunt down the commits that carry a
+  described fix, and merge that fix into the current branch. Trigger on
+  /merge-fix <description of the fix> or when the operator asks to find a fix
+  that lives on another branch and bring it over. Portable across projects.
+---
+
+The free text after `/merge-fix` is the fix description, exactly like `/loop`
+takes its prompt. Everything after the skill name is the search intent; there is
+no other input. If it is missing, ask for it and stop.
+
+## 1. Refuse to work on a dirty tree
+
+`git status --porcelain` must be empty. If it is not, stop and let the operator
+commit or open a worktree. Never stash, never `git checkout -- <file>`, never
+`git restore --staged`: those discard uncommitted work that other agents in the
+same tree may own.
+
+## 2. Fetch everything
+
+```
+git fetch --all --prune --tags
+```
+
+That is the "pull all branches" step. Do not check branches out one by one; all
+searching below runs against the fetched refs.
+
+## 3. Turn the description into search terms
+
+Derive from the operator's text: 2-4 keywords for commit messages, plus any
+symbol, string, or path the fix must have touched. State the derived terms in
+one line so a wrong reading is visible before the search.
+
+## 4. Search every ref
+
+Run all three; they find different things:
+
+```
+git log --all --oneline --regexp-ignore-case --grep='<keyword>'
+git log --all --oneline -S'<symbol or literal>'
+git log --all --oneline -- '<path or glob>'
+```
+
+Then reduce the hits:
+
+- Drop anything already merged: `git merge-base --is-ancestor <sha> HEAD`
+  exits 0 → the fix is already in, say so and stop.
+- Locate the rest: `git branch -a --contains <sha>`.
+- Read the actual change: `git show --stat <sha>` and the diff. A matching
+  commit subject is a lead, not evidence; the diff decides.
+
+## 5. Present the candidates
+
+One table: sha, branch(es), subject, files touched, one line on what the diff
+really does. Let the operator pick unless exactly one candidate survives step 4
+and its diff plainly is the described fix.
+
+## 6. Bring it over
+
+- One or a few commits, rest of the branch unwanted: `git cherry-pick -x <sha>…`
+  (`-x` records the origin).
+- The whole branch is wanted: `git merge --no-ff <branch>`.
+- Conflicts: resolve them by reading both sides. Never `-X ours`/`-X theirs`,
+  never `git checkout --ours/--theirs` wholesale — that silently drops one
+  side's fix. Unresolvable → `git cherry-pick --abort` / `git merge --abort`,
+  report what conflicted, change nothing else.
+
+## 7. Report
+
+Show `git log --oneline <base>..HEAD` and the resulting diffstat, then apply the
+confidence skill: what came over, what did not, what still has to be verified.
+Run the project's test or quality gate only when the operator asks for it.
+
+## Rules
+
+- Never push and never force anything; the merge stays local until the operator
+  ships it.
+- Never rewrite history to make a cherry-pick fit (no rebase of the target
+  branch, no amend of foreign commits).
+- If the search finds nothing, say so plainly and list the terms tried. Do not
+  reimplement the fix under this skill — that is a different task.