ci: run the AI self-review check on pull_request

GitHub blocks pull_request_target by default in public repositories from
December 2, 2026. The ai-self-review workflow now runs on pull_request.
It still checks out and runs the check script of the base branch and no
other code of the pull request. The workflow file itself now comes from
the pull request, so a pull request that changes it can weaken its own
check. .github/CODEOWNERS therefore makes a maintainer the code owner of
the workflows, CODEOWNERS and the check script; this is enforced once the
branch protection of main requires a review from code owners.

Additional changes
- Record the switch in ADR 0002, which supersedes the pull_request_target
  consequence of ADR 0001.
- Skip the comment step for pull requests from forks, whose token is
  read-only, and point their error message to the job summary only.
- Describe the trigger, code owners and fork limits in docs/develop/ai.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Assisted-by: Claude Code (claude-opus-5-5)

.github/workflows/ai-self-review.yml

@@ -3,19 +3,21 @@ name: ai-self-review
 # Checks the self-review declaration of agent-assisted pull requests, see
 # docs/adr/0001-self-declared-sha-bound-review-gate.md and docs/develop/ai.md.
 #
-# pull_request_target runs the workflow and the check script of the base branch, so a pull
-# request cannot weaken its own check. The job never checks out or runs code of the pull
-# request; it reads the body and the commit messages through the API only.
+# It runs on pull_request, see docs/adr/0002-self-review-check-on-pull-request.md. The workflow
+# file therefore comes from the pull request, but the job checks out and runs the check script of
+# the base branch; it never checks out or runs other code of the pull request and reads the body
+# and the commit messages through the API only. A pull request that changes this workflow can
+# weaken its own check, so .github/CODEOWNERS requires a maintainer's review for it.
 #
 # It runs only for pull requests into main, never on pushes to main or to other branches, and not
 # for pull requests into other branches.
 #
 # On failure it posts one comment that explains the fix, found again by its marker and updated in
-# place; once the check passes, the comment is updated to say so. Commenting needs
-# pull-requests: write, which is safe here because no pull request code runs.
+# place; once the check passes, the comment is updated to say so. Pull requests from forks get a
+# read-only token, so the comment step is skipped for them; the report is in the job summary.
 
 on:
-  pull_request_target:
+  pull_request:
     types: [opened, edited, synchronize, reopened]
     branches: [main]
 
@@ -83,6 +85,8 @@ jobs:
         echo "status=$status" >> "$GITHUB_OUTPUT"
         cat "$RUNNER_TEMP/report.md" >> "$GITHUB_STEP_SUMMARY"
     - name: Comment on the pull request
+      # The token of a pull request from a fork is read-only and cannot comment.
+      if: github.event.pull_request.head.repo.full_name == github.repository
       # A failing comment must not change the result of the check.
       continue-on-error: true
       env:
@@ -104,8 +108,13 @@ jobs:
       if: always()
       env:
         STATUS: ${{ steps.check.outputs.status }}
+        FROM_FORK: ${{ github.event.pull_request.head.repo.full_name != github.repository }}
       run: |
         if [ "${STATUS:-1}" != "0" ]; then
-          echo "::error::The self-review check failed; see the step summary or the pull request comment."
+          if [ "$FROM_FORK" = "true" ]; then
+            echo "::error::The self-review check failed; see the job summary of this run."
+          else
+            echo "::error::The self-review check failed; see the job summary or the pull request comment."
+          fi
         fi
         exit "${STATUS:-1}"