chore: notify Slack for SDK release PRs

[dhruva-vapi]

Linear ticket

https://linear.app/vapi/issue/PRISM-1030

Value

vapi employees can now get a Slack alert when Fern opens an SDK release PR, instead of relying on someone to notice the PR manually.

This supports the new manual approval path: Fern opens a PR, Slack tells the release channel, a human reviews and merges it, then a human publishes the release tag.

Evidence of value

• Adds a repo-local pull_request workflow for Fern-looking SDK release PRs.
• Sends the repo, PR, author, package, detected version, and release runbook link to #production-release-observability.
• Uses the existing PRODUCTION_RELEASE_OBSERVABILITY_SLACK_WEBHOOK repository secret.

API Changes (including webhooks + events)

• Is this changing the public API?

  • Yes
  • No

• If yes, is it backward-compatible?

  • Yes
  • No

Non backward-compatible changes might break customers' agents. Please proceed with care and notify the team.

Testing plan

• Parsed the workflow YAML locally.
• Confirmed the Slack webhook secret exists on the target SDK repo before opening the PR.
• After merge, the verification signal is a Slack message in #production-release-observability when Fern opens or reopens an SDK release PR.
• Regression signal: a Fern SDK release PR opens but no Slack message appears within a minute.