cleanup --json: a pre-upgrade control session in the project's own registry is reported in sessions.legacy_leftovers #917

bmad-loop cleanup --json reports a pre-upgrade bmad-loop-ctl-<16 hex> control session found in the project's own registry inside sessions.legacy_leftovers. #912 added that scan (runs.legacy_registry_leftovers merges superseded_ctl_sessions into the same grouped map the legacy-registry pass fills, and cmd_cleanup flattens it into legacy_leftovers).

The schema-2 description of that field is narrower: "session NAMES … that a legacy multiplexer registry still holds and that cleanup deliberately did not remove" (documents.cleanup_document). The project's current registry is not a legacy one. The machine.py contract says a change in what a value means needs a CLEANUP_SCHEMA_VERSION bump, and CLEANUP_SCHEMA_VERSION is still 2.

Impact

A consumer that reads only legacy_leftovers (and not the additive legacy_kill_commands beside it) can no longer tell a migration remainder from a control session standing in the current registry. The text mode is not affected: it labels each group with its registry, and the pre-upgrade session gets its own stderr line.

Options

• Report the own-registry session in a new additive field (for example sessions.superseded_ctl_sessions) and keep it out of legacy_leftovers. No version bump.
• Or keep it where it is, document the wider meaning, and bump CLEANUP_SCHEMA_VERSION to 3.

The first keeps existing consumers' reading of legacy_leftovers intact and is the smaller change for them. It touches the cleanup text mode, the TUI toast and the tests #912 and #916 added for this session.

Raised in the review of #916. Refs #912, #916, #733