Remove artifact-attachment backfill migration + mj-cli artifacts reclassify before next LTS baseline

At the next LTS build (end of May 2026), MJ will publish a new `B*__Baseline` script that becomes the install starting point. Versioned migrations dated earlier than that baseline will no longer run on fresh installs. At that point, the artifact-attachment backfill in `V202605190912__v5.35.x__Artifact_Attachment_Unification.sql` (Phase A: attachment → artifact backfill, Phase B: JSON-fallback reclassification) becomes dead code.

We need to remove:

- The `mj-cli artifacts reclassify` command (`packages/MJCLI/src/commands/artifacts/reclassify.ts` and its registration) — that command exists only to let operators re-run the same backfill logic against rows the migration left untouched (unregistered MIMEs at install time). Once the baseline ships with the unification schema and the seeded ArtifactType registry in place, no install would ever land in the state `reclassify` was built to repair.

Quote from review (PR #2569):

> I suppose it is ok to keep for now, but when we do our first LTS build at end of May we'll be shipping a new B script == new Baseline which means that old migration won't run anymore. At that point, we need to remove this - so add an issue to GH to flag this so we don't lose track of removing this later.

Trigger: the PR that introduces the new B-script baseline (end-of-May 2026 LTS). Block on landing this cleanup in the same release.