ci: comment when stale-contributor.yml applies closing-soon

[chaodu-agent]

Summary

Found while investigating why PR #1272 had the closing-soon label despite
already having a valid Discord Discussion URL in its body. Turned out
pr-discussion-check.yml (the Discord URL check) wasn't the cause at all —
it ran once, matched the URL, and cleared the label correctly. The label
kept reappearing because of a separate, unrelated workflow:
stale-contributor.yml, which adds closing-soon to any PR that's held the
pending-contributor label for more than 2 days — with no explanation
posted to the author, unlike pr-discussion-check.yml's Discord-URL check,
which always posts a [!CAUTION] comment.

That silence is the actual problem this PR fixes: an author (or anyone
investigating) sees closing-soon appear with zero context about which of
the several closing-soon-related workflows applied it or why, since there's
no comment to check.

Change

Add a caution comment to stale-contributor.yml, matching
pr-discussion-check.yml's existing alert style, whenever it adds
closing-soon:

> [!CAUTION]
> This PR has been waiting on the author for more than 2 days
> (labeled `pending-contributor` since <date>).
> It will be **automatically closed in 24 hours** if there is no update.

Please push a commit, respond to outstanding review feedback, or comment
here to keep this PR open. Feel free to reopen a new PR later if it gets
closed and you want to pick it back up.

Uses a <!-- openab-stale-contributor-check --> marker (checked against
existing comments) to avoid duplicate comments across runs — belt-and-suspenders
alongside the existing early-continue guard for PRs that already carry
closing-soon.

Scope

Only stale-contributor.yml (PRs) was changed, per the specific ask.
stale-issue.yml has the identical gap on the issue side but wasn't touched
here — happy to open a follow-up for it if wanted.

Testing done

• Validated the workflow YAML parses correctly.
• Not dry-run against a real stale PR (would require waiting 2 real days or
  manipulating label-event timestamps in a test repo) — the added logic
  mirrors pr-discussion-check.yml's already-working comment-posting pattern
  closely enough that I'm confident in the mechanics, but flagging this as
  the one part that's unverified against a live run.

[chaodu-agent]

LGTM ✅ — Adds a clear caution comment to stale-contributor.yml so authors know why closing-soon appeared.

What This PR Does

When stale-contributor.yml adds the closing-soon label to a PR that has been pending-contributor for >2 days, it now posts an explanatory [!CAUTION] comment — matching the style already used by pr-discussion-check.yml. Previously, the label appeared with no context.

How It Works

After adding the closing-soon label, the workflow fetches existing comments and checks for an HTML marker (<!-- openab-stale-contributor-check -->). If no marker comment exists, it posts one explaining the situation and what the author should do.

Findings

#	Severity	Finding	Location
1	🟢	Clean dedup pattern with HTML marker — mirrors the proven pr-discussion-check.yml approach	.github/workflows/stale-contributor.yml:55
2	🟢	Message content is clear, actionable, and matches existing project tone	.github/workflows/stale-contributor.yml:65-77
3	🟢	Minimal, focused change — only touches the file that needs it	—

Finding Details

🟢 F1: Clean dedup pattern

Uses the same <!-- marker --> + comments.some() approach proven in pr-discussion-check.yml. Belt-and-suspenders alongside the existing if (labels.includes(CLOSING)) continue guard.

🟢 F2: Clear message content

The caution block tells the author exactly what happened, when it happened (pending-contributor since date), and what to do — push a commit, respond to feedback, or comment.

🟢 F3: Focused scope

Only stale-contributor.yml is touched. The PR description transparently notes the same gap exists in stale-issue.yml and offers a follow-up.

Baseline Check

• PR opened: 2025-07-04
• Main already has: stale-contributor.yml that silently adds closing-soon; pr-discussion-check.yml that posts explanatory comments
• Net-new value: Bridges the UX gap — stale-contributor.yml now explains itself like the other workflow does

What's Good (🟢)

• Mirrors a proven pattern from pr-discussion-check.yml rather than inventing something new
• HTML marker prevents duplicate comments across multiple hourly runs
• PR description is thorough — clearly explains the investigation that led to the fix and acknowledges testing limitations honestly
• Scope is intentionally minimal with a follow-up offered for stale-issue.yml

Robustness Note (not blocking)

If the createComment API call fails after the label is already added, subsequent hourly runs will skip the PR (because closing-soon is already present), so the explanatory comment would never be posted. In practice this is unlikely and the label itself still serves its purpose, but a future hardening pass could swap the order (comment first, then label) or remove the closing-soon early-exit for the comment-posting logic. Not blocking since the existing workflow has the same structural pattern.