fix: Pin actions/stale to full SHA

[charlesvien]

Problem

The workflow is failing to run because the action is not pinned.
https://github.com/PostHog/code/actions/runs/26611117555/job/78417017648

Changes

• Pinned actions/stale to the full sha

How did you test this?

I did not

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

[greptile-apps]

T-Rex Logs

What T-Rex did

• Verified the pinned actions/stale commit exists and resolves successfully.
• Confirmed the v9 tag points to the same commit SHA used in the workflow.
• Confirmed the pinned action metadata declares every input used by .github/workflows/stale.yml.
• Parsed the workflow YAML successfully.

Artifacts

Commit lookup for pinned actions/stale SHA

• Helps verify the pinned commit can be located and resolves correctly.

Tag lookup showing v9 resolves to the pinned SHA

• Shows that the v9 tag points to the same commit used by the workflow.

Tag list showing v9 and v9.1.0 point to the pinned SHA

• Demonstrates that both v9 and v9.1.0 reference the same pinned commit.

Pinned action metadata with configured inputs

• Documents the inputs declared by the pinned action metadata.

_{Ran code and verified through T-Rex}

_{Reviews (1): Last reviewed commit: "Pin actions/stale to full SHA" | Re-trigger Greptile}

[stamphog]

This PR modifies a GitHub Actions workflow file, which is classified as CI/infra and requires human review before merging.