tool-build: build MyDriftWatcher (flag cross-repo convention drift)

[lorenzoliuzzo]

Part of the fleet milestone (#1 §2). Design: mythings-core/docs/tools/my-drift-watcher.md.

MyDriftWatcher flags cross-repo convention drift — timely now that the org is
20+ repos and drift is already real (this milestone's review found stale local
branches and a genesis README left as the raw my-template stub).

Scope (v0)

• Deterministic: diff each repo's vendored HARNESS.md, CI shape, pyproject
  seams, and required docs banners against the canonical mythings-core.
• One Engine call only for the summary/priority of drift found.
• Output: a drift report issue per offending repo (or an org roll-up).

Steps

• Scaffold + implement the deterministic checks + one summary Engine call.
• Tests over a fixture repo that intentionally drifts.