feat(ci): skip linear comments for non-stable releases

[Piotr1215]

Closes OPS-394

Allow stable releases to add comments on already-released issues; pre-releases skip if already released.

---

Note

Distinguishes stable vs pre-release tags in Linear sync to skip comments for pre-releases on already-released issues, add a "now available in stable" comment for stable tags, and updates tests accordingly.

• Linear sync (hack/linear-sync/linear.go):
  • Add isStableRelease(version) to detect stable versions.
  • Update MoveIssueToState(...):
    • If already in Released and tag is pre-release: skip entirely.
    • If already in Released and tag is stable: add "Now available in stable" comment without state change.
    • Otherwise require Ready for Release before moving to Released; always add appropriate release comment.
• Tests (hack/linear-sync/linear_test.go):
  • Add TestIsStableRelease and TestStableReleaseCommentText.
  • Maintain/mock helpers and filtering tests for move logic.

^{Written by Cursor Bugbot for commit 30e67c4. This will update automatically on new commits. Configure here.}

[Piotr1215]

Looks like flaky e2e, 137 is usually OOMKill, so it's unrelated to the PR change.