*: add skill for cherry-pick PR repair

.agents/skills/fix-cherry-pick-pr/SKILL.md

@@ -0,0 +1,141 @@
+---
+name: fix-cherry-pick-pr
+description: Repair cherry-pick pull requests in tikv/pd when automated cherry-picks leave committed conflict markers, drift from the source PR, or need parity verification against the original PR. Use when given a source PD PR and its cherry-pick PR, or a cherry-pick PR that references an original PR, and asked to compare diffs, resolve release-branch cherry-pick conflicts, run failpoint-aware verification, and push the fixed cherry-pick branch.
+---
+
+# Fix Cherry-Pick PR
+
+## Overview
+
+Repair and verify cherry-pick PRs in `tikv/pd`.
+Compare the source PR and the cherry-pick PR first, then fix the cherry-pick branch without losing release-branch-only code.
+
+## 1. Identify the PR pair
+
+- Inspect the cherry-pick PR with `gh pr view <pr> --repo tikv/pd --json number,title,body,baseRefName,headRefName,headRepositoryOwner,url,commits`.
+- Extract the source PR number from one of:
+  - a title suffix like `(#10131)`
+  - body text like `This is an automated cherry-pick of #10131`
+  - the head commit message when the body is ambiguous
+- Read the linked issue if present to confirm the real bug and expected behavior.
+- Treat GitHub `mergeable` status as insufficient. A PR can be mergeable while still containing committed conflict markers in the patch.
+
+## 2. Compare source and cherry-pick diffs before editing
+
+- Compare changed files first:
+
+```bash
+gh pr diff <source-pr> --repo tikv/pd --name-only
+gh pr diff <cherry-pick-pr> --repo tikv/pd --name-only
+```
+
+- Compare the final aggregate diff, not raw commit count. Cherry-pick PRs can have extra cleanup commits.
+- Normalize diff output when checking parity:
+
+```bash
+diff -u \
+  <(gh pr diff <source-pr> --repo tikv/pd | awk '
+      /^diff --git a\// { file = $4; sub("^b/", "", file); next }
+      /^\+\+\+ b\// { file = substr($0, 7); next }
+      /^--- a\// { next }
+      /^[+-][^+-]/ && file != "" { print file ":" $0 }
+    ' | sort) \
+  <(gh pr diff <cherry-pick-pr> --repo tikv/pd | awk '
+      /^diff --git a\// { file = $4; sub("^b/", "", file); next }
+      /^\+\+\+ b\// { file = substr($0, 7); next }
+      /^--- a\// { next }
+      /^[+-][^+-]/ && file != "" { print file ":" $0 }
+    ' | sort)
+```
+
+- Accept differences caused only by base-branch context:
+  - different line numbers or surrounding unchanged context
+  - branch-only helper or API definitions that must remain in the release branch
+  - different deleted lines when the release branch had already diverged before cherry-pick
+- Do not accept:
+  - committed `<<<<<<<`, `=======`, `>>>>>>>` markers
+  - missing behavior from the source PR
+  - extra functional changes not required by the release branch
+
+## 3. Prepare the working branch
+
+- Keep the current worktree clean before switching.
+- Fetch the cherry-pick head branch from its actual remote, often `tichi` or `ti-chi-bot`.
+- Create a local working branch that tracks the cherry-pick head.
+- Confirm the expected head commit before editing.
+
+Example:
+
+```bash
+git fetch tichi cherry-pick-10131-to-release-8.5
+git switch -c pr-10301-fix-conflict-markers --track tichi/cherry-pick-10131-to-release-8.5
+```
+
+## 4. Repair the cherry-pick patch
+
+- Search touched files for conflict markers:
+
+```bash
+rg -n '<<<<<<<|=======|>>>>>>>' <touched-files>
+```
+
+- Resolve each block by reconstructing the final intended code. Do not blindly keep one side.
+- Preserve three things at once:
+  - the source PR's intended behavior
+  - release-branch-only code that existed before cherry-pick and must remain
+  - branch-specific adjacent APIs, helpers, or tests
+- When the automated cherry-pick inserted conflict markers around an existing release-branch API, keep the API and insert the source PR logic beside it in the correct final location.
+- If the cherry-pick introduced no real semantic drift beyond committed markers, keep the follow-up commit minimal. Remove markers and restore the intended final code only.
+
+## 5. Verify in PD with failpoint discipline
+
+- Follow PD's failpoint rules. If the touched tests rely on failpoints, do not rely on plain `go test` results before instrumentation.
+- Prefer the narrowest targeted verification first. For the store-limit workflow used in `#10131` and `#10301`:
+
+```bash
+make failpoint-enable
+go test ./server/cluster -run 'TestCheckCache|TestStoreLimitChangeRefreshLimiter' -count=1 -v -timeout=2m
+make failpoint-disable
+```
+
+- If you need broader signal, run the narrowest additional package or integration test target that covers the touched files.
+- If that extra verification fails in unrelated packages, separate that from the cherry-pick fix. Report the failing package and test name rather than attributing it to the cherry-pick automatically.
+- Before finishing, confirm:
+
+```bash
+rg -n '<<<<<<<|=======|>>>>>>>' <touched-files>
+git diff -- <touched-files>
+git status --short --branch
+```
+
+- If failpoint-generated files appear after an interrupted run, restore them with `make failpoint-disable` before committing or doing other non-test work.
+
+## 6. Commit and push
+
+- Use a focused follow-up commit that explains the cleanup, for example:
+
+```bash
+git commit -s -m 'storelimit: resolve cherry-pick markers in #10301'
+```
+
+- Push `HEAD` back to the existing cherry-pick head branch if you have permission.
+- If push fails and the user did not specify a fallback, stop and report the push error instead of inventing a new branch plan.
+
+## 7. Report the result
+
+Include:
+
+- source PR number and cherry-pick PR number
+- whether the final cherry-pick diff matches the source PR semantically
+- whether any extra behavior was introduced
+- files repaired
+- targeted tests run and their result
+- any unrelated broader test failure that remains outside the cherry-pick fix
+
+## Common pitfalls
+
+- Do not equate GitHub `MERGEABLE` with "patch is clean".
+- Do not compare raw commit lists. Compare final diffs.
+- Do not delete release-branch-only code when resolving a cherry-pick block.
+- Do not leave failpoint-generated files in the working tree.
+- Do not claim parity until you compare the final aggregate diffs after the fix.

AGENTS.md

@@ -143,6 +143,7 @@ Reusable agent skills live under `.agents/skills/`. Each skill has a `SKILL.md`
 
 | Skill | Purpose | Prerequisites | Docs |
 |---|---|---|---|
+| `fix-cherry-pick-pr` | Repair cherry-pick PRs by comparing source and cherry-pick diffs, resolving committed conflict markers, preserving release-branch-only code, and running failpoint-aware verification. | `gh` CLI authenticated with tikv/pd repo access; git remotes for the source repo and cherry-pick branch; PD test environment able to run failpoint-aware verification. | [`.agents/skills/fix-cherry-pick-pr/SKILL.md`](.agents/skills/fix-cherry-pick-pr/SKILL.md) |
 | `create-pr` | Push the current branch and open a well-formatted PR on tikv/pd. Analyzes commits, generates PR title/body following the repository template, and submits via `gh pr create`. | `gh` CLI authenticated with tikv/pd repo access; local commits on a non-master branch. | [`.agents/skills/create-pr/SKILL.md`](.agents/skills/create-pr/SKILL.md) |
 
 ## Microservices / NextGen