[CI] claude_review: target source PR's branch for follow-up PRs (#13774)

* [CI] claude_review: target source PR's branch for follow-up PRs

The follow-up PR was always cut from main, so once main moved on from the
PR's base the cherry-pick conflicted and the run failed (see run
26191835696). For non-fork PRs we now target the PR's own head branch
instead — Claude's edits apply cleanly regardless of how main has diverged,
and merging the follow-up folds them into the original PR. Fork PRs still
target the default branch since we can't push to a fork.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* [CI] claude_review: skip COMMIT THIS on fork PRs

Falling back to main as the base for fork PRs hits the same cherry-pick
conflict pattern the previous commit fixed for source PRs, and we can't
push to the fork's branch anyway. Bail early with a friendly comment
pointing users to apply Claude's suggestions manually or open an issue.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/claude_review.yml

@@ -156,7 +156,6 @@ jobs:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           PR_NUMBER: ${{ github.event.issue.number || github.event.pull_request.number }}
           COMMENT_USER: ${{ github.event.comment.user.login }}
-          BASE_BRANCH: ${{ github.event.repository.default_branch }}
         run: |
           set -euo pipefail
 
@@ -186,11 +185,18 @@ jobs:
             exit 0
           fi
 
-          # For fork PRs, an earlier step redirected `origin` to a local bare
-          # repo to sandbox claude-code-action. Undo that redirect so our push
-          # reaches the real base repo. Safe: only Claude's edits within the
-          # allowed paths are committed below — never the fork's other changes.
-          git config --unset-all url."file:///tmp/local-origin.git".insteadOf 2>/dev/null || true
+          PR_INFO=$(gh pr view "$PR_NUMBER" --json headRefName,isCrossRepository)
+          PR_BRANCH=$(echo "$PR_INFO" | jq -r '.headRefName')
+          IS_FORK=$(echo "$PR_INFO" | jq -r '.isCrossRepository')
+
+          # COMMIT THIS isn't supported on fork PRs: we can't push to the
+          # fork's branch, and falling back to main almost always conflicts
+          # once the PR touches files that also moved on main. Bail early —
+          # Claude's review comment with the suggested diff still stands.
+          if [[ "$IS_FORK" == "true" ]]; then
+            post_status "ℹ️ \`COMMIT THIS\` isn't supported on fork PRs. Apply Claude's suggestions manually, or open an issue to track them. See [workflow run]($RUN_URL)."
+            exit 0
+          fi
 
           git config user.name "claude[bot]"
           git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
@@ -208,8 +214,6 @@ jobs:
             exit 1
           fi
 
-          PR_BRANCH=$(gh pr view "$PR_NUMBER" --json headRefName --jq '.headRefName')
-
           if [[ "$PR_BRANCH" == claude/pr-* ]]; then
             # Source PR is already a Claude-opened PR — iterate in place by
             # committing and pushing straight to its head branch instead of
@@ -222,9 +226,14 @@ jobs:
             exit 0
           fi
 
-          # Otherwise: commit on the source PR's branch to get a clean SHA,
-          # then cherry-pick onto a fresh branch cut from the default branch.
-          # The follow-up PR's diff is therefore exactly Claude's edits vs. main.
+          # Target the source PR's head branch. The follow-up then applies
+          # cleanly regardless of how main has diverged, and merging it lands
+          # Claude's edits onto the PR for the maintainer to fold in.
+          BASE_BRANCH="$PR_BRANCH"
+
+          # Commit on the source PR's branch to get a clean SHA, then
+          # cherry-pick onto a fresh branch cut from BASE_BRANCH so the
+          # follow-up PR's diff is exactly Claude's edits vs. BASE_BRANCH.
           NEW_BRANCH="claude/pr-${PR_NUMBER}-$(date -u +%Y%m%d-%H%M%S)"
 
           git commit -m "Apply changes from Claude (requested by @${COMMENT_USER} on #${PR_NUMBER})
@@ -248,6 +257,6 @@ jobs:
             --title "Apply Claude's changes from #${PR_NUMBER}" \
             --body "Automated PR with edits Claude made in response to \`COMMIT THIS\` from @${COMMENT_USER} on [#${PR_NUMBER}](${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/pull/${PR_NUMBER}).
 
-          Targets \`${BASE_BRANCH}\` — independent of #${PR_NUMBER}. Further \`COMMIT THIS\` requests on *this* PR will commit directly to it.")
+          Targets \`${BASE_BRANCH}\` (the head branch of #${PR_NUMBER}). Merging this brings Claude's edits into that PR.")
 
           post_status "✅ Opened follow-up PR (into \`${BASE_BRANCH}\`) with Claude's edits: ${NEW_PR_URL}"