ci: add the empty changeset the gate requires (#4251)

The PR changes tooling and CI only — no publishable package diff — which is
exactly the case the Check Changeset gate names as an empty changeset.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AWW5xEALZNU5VBXfGyWPGz

Author: claude

.changeset/slot-lookup-ratchet-enforcement.md

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+---
+---
+
+ci(tooling): enforce the #4251 slot-lookup sweep ratchet with a counted baseline
+
+The grandfather list shipped as paths in `eslint.config.mjs` under a comment
+reading "NEVER add an entry" — a promise nothing checked. ESLint cannot express
+a ratchet: an ignored file is ignored completely, so a NEW `getService<any>(…)`
+added to an already-listed file rode the old entry with `pnpm lint` green.
+
+The list moves to `scripts/slot-lookup-baseline.json` (file → site count) and
+becomes the single source: its keys are the config's `ignores`, its values are
+what `pnpm check:slot-lookup` enforces. The script re-runs ESLint with the
+grandfathering lifted and matches reports by the rule's exact message, so the
+counter cannot drift from the rule. It fails on a new file, a grown count, a
+stale entry, and — via a key-set comparison against the merge base with main —
+on a file being ADDED to the list, which no count check can see.
+
+Tooling and CI only; releases nothing.