docs(engineer-bot): clarify no_change outcome + coverage_pr_url key are correct

Two review findings (raised repeatedly) assumed the workflow was wrong; both are
false positives verified against the engine source. Add clarifying comments so
they don't recur:

- `case ... no_change)`: the engine maps the agent's `no_change_needed` to the
  emitted $GITHUB_OUTPUT value `no_change` (run_author.py), so the arm is correct.
- `coverage_pr_url`: engineer_bot.publish emits this key for ALL flows (legacy
  name), not just coverage — correct for bug-fix too.

Co-authored-by: Isaac

Author: eric-wang-1990

.github/workflows/engineer-bot.yml

@@ -169,11 +169,19 @@ jobs:
           REPO: ${{ github.repository }}
           ISSUE: ${{ steps.ctx.outputs.issue_number }}
           OUTCOME: ${{ steps.author.outputs.outcome }}
+          # `coverage_pr_url` is the engine publish module's PR-URL output key for
+          # ALL flows (bug-fix included), not just coverage — the name is legacy.
+          # engineer_bot.publish emits this key regardless of flow, so it is correct here.
           PR_URL: ${{ steps.publish.outputs.coverage_pr_url }}
           REASON: ${{ steps.author.outputs.reason }}
           RUN_URL: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}
         run: |
           BODY="$RUNNER_TEMP/issue_comment.md"
+          # $OUTCOME is the engine's emitted outcome (steps.author.outputs.outcome),
+          # NOT the agent's internal enum. The engine maps the agent's
+          # `no_change_needed` (what the prompt asks the agent to report) to the
+          # `$GITHUB_OUTPUT` value `no_change` (run_author.py). So `no_change)` below
+          # is the correct arm — do not "fix" it to `no_change_needed)`.
           case "$OUTCOME" in
             success)
               if [ -n "$PR_URL" ]; then