v1 Unit 8: Retire legacy

[mairas]

Part of the shared-workflows v1 plan (tracking issue: issue 49).

Goal: Remove frozen files and the hatlabs copy.

Requirements: R1, R2

Dependencies: Unit 7 verification

Files:

• Delete: .github/workflows/build-release.yml, .github/workflows/pr-checks.yml, .github/workflows/publish-stable.yml

Approach: Ship as v1.1.0 (no pinned caller uses the deleted files). Archive hatlabs/shared-workflows with approval at that time. Close issue 29, issue 48, hatlabs issue 2, issue 41 via the PRs that fix them.

Test expectation: none -- deletion; inventory from Unit 7 proves no references.

Verification: Inventory shows no caller of deleted files; hatlabs repo archived.