scripts: add merge-branches Claude Code skill

.claude/commands/merge-branches.md

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+---
+description: Merge a source branch into a destination branch via scripts/merge_branches.py, walk through each conflict interactively, commit as fix_conflicts, and post inline PR review comments tagging the authors who caused each conflict.
+---
+
+# Merge Branches
+
+Run a branch-merge end-to-end: kick off `scripts/merge_branches.py`, resolve each conflict interactively with the user, build-verify, commit, push, and document every resolution as an inline review comment on the resulting PR. Follow the steps in order. Do not batch-skip steps — interactivity is the point.
+
+## Background
+
+- The merge script `scripts/merge_branches.py` creates a fresh branch off the destination, merges the source, and pushes a PR with conflict markers committed in-place. It also adds a comment to the PR linking the conflicting files on each side.
+- This skill picks up from that point: it resolves the conflict markers, verifies the build, commits, and posts per-conflict inline review comments tagging the people whose changes collided.
+- Graphite (`gt`) is the project's git wrapper — use it for commits and pushes.
+
+---
+
+## Step 1: Gather inputs
+
+Ask the user (use `AskUserQuestion`):
+
+1. **Source branch** (the branch being merged *from*, e.g. `main-v0.14.2`).
+2. **Destination branch** (the branch being merged *into*, e.g. `main`). If the source has a default in `scripts/merge_paths.json`, offer it as a recommended option.
+3. **Use a worktree?** Recommend yes — merges leave the local branch in an unusual state. If yes, use `EnterWorktree` (creating one based on `origin/<dst>` via `git worktree add` if a custom base ref is needed).
+
+Confirm the choices in one sentence before proceeding.
+
+---
+
+## Step 2: Fetch and run the merge script
+
+```bash
+git fetch origin <src-branch> <dst-branch>
+python3 scripts/merge_branches.py --src <src-branch> --dst <dst-branch>
+```
+
+Capture the script's output. From it, extract:
+- The new merge branch name (`<user>/merge-<src>-into-<dst>-<timestamp>`).
+- The PR URL / number created by `gh pr create`.
+- The list of conflicted files (printed under `git status -s | grep -E ...`).
+
+If the script ran with `gt`-untracked state, run `gt track --parent <dst-branch>` so subsequent `gt modify`/`gt submit` work.
+
+---
+
+## Step 3: Enumerate conflicts
+
+Use `grep` to find conflict markers in every reported file:
+
+```bash
+grep -n "<<<<<<\|>>>>>>\|||||||" <files>
+```
+
+For each conflict region, capture:
+- The file and line range.
+- The **HEAD** (destination) section.
+- The **base** section (between `|||||||` and `=======`) — `git config merge.conflictstyle diff3` is set by the script, so the merge base is shown explicitly.
+- The **incoming** (source) section.
+
+Also count conflicts per file so the user knows how many decisions to expect.
+
+---
+
+## Step 4: Identify authors for each conflict
+
+For each conflicting hunk, identify who introduced each side so they can be tagged later. Run on the merge-base:
+
+```bash
+MERGE_BASE=$(git merge-base origin/<src> origin/<dst>)
+# Author of dst-side change:
+git log --format='%h %an <%ae> %s' -S '<identifying-token>' "$MERGE_BASE..origin/<dst>" -- <file>
+# Author of src-side change:
+git log --format='%h %an <%ae> %s' -S '<identifying-token>' "$MERGE_BASE..origin/<src>" -- <file>
+```
+
+Pick a distinctive token from each side of the conflict (a new identifier, a removed crate name, etc.). Look up the GitHub handle of the author by reading the PR (`mcp__github__pull_request_read` with the `#NNNN` from the commit subject) and capturing `user.login`.
+
+Record `{ file, line, side, author_handle, pr_number }` for each conflict — this drives Step 7.
+
+---
+
+## Step 5: Resolve conflicts one by one
+
+For **each** conflict region (in file order, top to bottom within a file):
+
+1. Show the user a tight summary:
+   - **File and line.**
+   - **What the destination side did** (and which PR / who).
+   - **What the source side did** (and which PR / who).
+   - **Proposed resolution** with reasoning. Common patterns:
+     - *Orthogonal changes* → keep both (e.g. two independent params added at the same position).
+     - *One side removed something now unused, the other added something now used* → drop the removed item, keep the added one. Verify "unused" with `grep` before claiming it.
+     - *Same semantic change in different terms* → pick one, ensure all call sites match.
+2. Use `AskUserQuestion` with options:
+   - **Accept** the proposed resolution.
+   - **Edit** / give feedback (free text via "Other").
+   - **Skip** — leave the conflict markers, come back later.
+3. On accept, apply the change with `Edit` (replace the entire `<<<<<<<` … `>>>>>>>` block, including markers, with the resolved text).
+4. After all conflicts in a file are resolved, verify no markers remain in that file.
+
+Do NOT batch all proposals before asking — propose, ask, apply, then move on. The user wants to think through each one.
+
+---
+
+## Step 6: Build-verify and catch silent merge skews
+
+Run `cargo check` on every crate that owns a conflicted file:
+
+```bash
+cargo check -p <crate>
+```
+
+Textual conflicts are only half the story — the merge driver doesn't catch **silent API skews** where one side renamed a type or changed a signature and the other side's text merged cleanly but no longer typechecks. If `cargo check` fails:
+
+1. Read the error.
+2. Identify which side introduced the breaking change (usually `main`-side refactors).
+3. Propose a fix to the user with the same Accept / Edit / Skip flow as Step 5.
+4. Record it as an additional "silent skew" entry for Step 7.
+
+Repeat until the build is clean.
+
+---
+
+## Step 7: Commit and push via Graphite
+
+```bash
+git add <resolved-files>
+gt modify -cam "fix_conflicts"
+gt submit --force
+```
+
+Use `--force` — the merge script already pushed the branch, so Graphite will refuse a normal submit. The local state is authoritative at this point.
+
+Confirm the PR was updated by re-checking `gt submit` output or `gh pr view <PR#>`.
+
+---
+
+## Step 8: Post per-file inline review comments
+
+Build a single PR review with one inline comment per conflict (and per silent-skew fix). Use `gh api` to POST to `/repos/{owner}/{repo}/pulls/{PR#}/reviews`. Each comment object needs:
+
+- `path` — repo-relative file path.
+- `line` — the line number of the resolved hunk in the **head** commit (the `fix_conflicts` commit). Use `git rev-parse HEAD` for `commit_id`.
+- `side: "RIGHT"`.
+- `body` — explain the conflict, the resolution, and cc the authors with `@<handle>`.
+
+Suggested comment template:
+
+```markdown
+**Conflict:** <one-line description>.
+- `<dst-branch>` (@<dst-author>, #<dst-pr>) <what they did>.
+- `<src-branch>` (@<src-author>, #<src-pr>) <what they did>.
+
+**Resolution:** <what we did and why>.
+
+cc @<dst-author> @<src-author>
+```
+
+For silent skews (Step 6), label them as **"Silent merge skew (not a textual conflict)"** so reviewers know why a comment landed somewhere with no conflict markers.
+
+Write the review payload to a tmp JSON file (because the bodies contain newlines and markdown that don't survive `-F` flags cleanly) and submit:
+
+```bash
+gh api repos/<owner>/<repo>/pulls/<PR#>/reviews \
+  --method POST --input /tmp/pr_review.json
+```
+
+The top-level `body` of the review should be a short header: *"Per-file conflict resolution notes (commit `fix_conflicts`). Source side: @<src-author>. Destination side: @<dst-author>. Inline notes below."* Set `event: "COMMENT"` (not `APPROVE` or `REQUEST_CHANGES`).
+
+Clean up `/tmp/pr_review.json` afterward.
+
+---
+
+## Step 9: Report back
+
+Tell the user:
+- PR URL.
+- Commit SHA of `fix_conflicts`.
+- Number of conflicts resolved + number of silent skews fixed.
+- Link to the posted review.
+
+Then stop. Do not merge the PR — that's a separate decision.
+
+---
+
+## Notes
+
+- **Never** mask a failing `cargo check` with `#[ignore]` or by deleting code — investigate the skew (Step 6).
+- **Never** amend the merge commit — keep `fix_conflicts` as a separate commit so reviewers can see the resolution diff cleanly.
+- If the user says **Skip** on any conflict, do not commit until they come back and resolve it — leaving conflict markers in a commit will break CI.
+- If `mcp__github__pull_request_read` is unavailable, fall back to `gh pr view <PR#> --json author` to fetch the GitHub handle.