feat(stale): reusable stale-PR workflow + name the repo in the Slack digest (BE-2776)

[mattmillerai]

ELI-5

Two repos (cloud and comfy-infra) each had their own copy of the same "stale PR bot" workflow, and both post to the same Slack channel. Their digests looked identical — a bare #158 doesn't say which repo it's from, so the two batches were impossible to tell apart. This adds one shared reusable version of that workflow to this repo (so there's no more copy-paste drift), and makes the Slack header name the source repo so each batch is unambiguous.

What this PR does

• Adds .github/workflows/stale.yml as a reusable (on: workflow_call). It ports the existing actions/stale + Slack chat.postMessage logic verbatim, parameterizing what differs per repo:
  • slack_channel (default C08V9NDB3B4), days_before_pr_stale/days_before_pr_close, stale_pr_message/close_pr_message, stale_pr_label, exempt_pr_labels, and a dry_run pass-through. SLACK_BOT_TOKEN is passed via secrets: (optional — the sweep still runs without it).
  • The caller owns the schedule + workflow_dispatch dry-run toggle (a reusable can't declare those triggers); the header documents the caller pattern.
• Names the repo in the digest header (*Stale PR bot — <owner/repo>*) so two repos' batches in the same channel are distinguishable. Per-PR link lines are unchanged — they already resolve via github.repository.
• Updates the README workflows table.

actionlint (+ shellcheck on the inline script) is clean.

Follow-ups (must land after this merges)

Per the "land the reusable first, then flip callers" sequencing (a caller referencing a not-yet-merged path fails), these are separate PRs in their own repos, pinned to this PR's merge SHA:

1. Replace Comfy-Org/cloud/.github/workflows/stale.yml with a thin caller.
2. Replace Comfy-Org/comfy-infra/.github/workflows/stale.yml with a thin caller.
3. Optionally add a bump-stale-callers.yml dispatcher (mirroring bump-cursor-review-callers.yml) to keep those callers pinned — deferred because the callers don't exist yet.
4. Confirm whether any other repo carries the same PR+Slack-digest stale bot (org search shows only cloud + comfy-infra match; the various stale-issues.yml are a different issue-triage bot, out of scope).

Judgment calls

• exempt_pr_labels default is cloud's superset (includes do-not-merge); comfy-infra's current copy omits it. Broadening to exempt do-not-merge is arguably an improvement (don't nag/close a deliberately-held PR), but the comfy-infra caller can override the input to its exact prior set if desired.
• The "auto-closes in N days" text now interpolates days_before_pr_close instead of a hardcoded "14"; the closed-digest line drops the "(~4 weeks inactive)" parenthetical since it was only accurate at the default 14/14 thresholds.

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📥 Commits

Reviewing files that changed from the base of the PR and between df507e6 and 36f24ca.

📒 Files selected for processing (2)

• .github/workflows/stale.yml
• README.md

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch matt/be-2776-stale-reusable

✨ Simplify code

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