Immutable launch links / tag policy

[Haserjian]

Context

Every public link in posts, README, and docs should point to an immutable tag (e.g., launch-2026-02-11), not main. This prevents data drift: if main advances and a scan study number changes, old links become misleading.

Current state

• Post links are pinned to launch-2026-02-11 tag as of commit 6195ab2.
• test_messaging_drift.py catches pinned SHA blobs but not main vs tag drift.

Next steps

• Document the tag policy: every public-facing link should use a release tag
• Add a drift guard test that warns if any post link points to main
• Consider using GitHub Releases instead of raw tags for better discoverability
• Automate: pre-post script that verifies all links resolve (HTTP 200)