ci: retire skill-drift.yml (no-op stub)

[castrojo]

What

Deletes .github/workflows/skill-drift.yml.

Why

This workflow calls projectbluefin/actions/.github/workflows/skill-drift-check.yml, which is a deprecated no-op stub. Its own header says so:

name: Skill Drift Check (deprecated no-op)
# Retained as a no-op stub so consumer repos calling it at @v1 do not break.
# Consumer repos should delete their skill-drift.yml on next maintenance pass.

Its only step echoes "Skill drift check removed. Delete skill-drift.yml from consumer repo." and exits 0 without inspecting any changed path.

So this workflow has been running on every PR and returning a silent green result regardless of what changed. A green check that proves nothing is worse than no check, because it reads as evidence that a skill update was not needed.

Five repos carried this: bluefin, bluefin-lts, dakota, knuckle, testsuite. common never wired it up.

Safety

skill-drift is not a required status check in this repo (verified against the ruleset), so removing it cannot stall a PR or the merge queue.

Policy

Per projectbluefin/common docs/factory/agentic-model.md, process conventions are enforced at developer time by pre-commit; CI may re-run that suite as a single aggregate step, and bespoke per-convention CI jobs are banned.

The obligation to update skills alongside implementation is unchanged — only the false claim that CI enforced it is gone.

Retirement record: https://github.com/projectbluefin/common/blob/main/docs/skills/skill-drift.md