Canonical health catalogue

Parent

Closes part of #281; parent remains open.

What to build

Make the extracted ToolRegistry the only source of the health tool catalogue and availability counts. The health endpoint must produce its legacy response from one coherent registry snapshot and must tolerate an individual probe failure without failing the complete health response.

Acceptance criteria

• The health route has no independent tool catalogue.
• Total, available, essential and category counts derive from one registry snapshot.
• The legacy health JSON field names and overall shape are preserved.
• A failed individual probe is represented as unavailable with diagnostic context.
• One failed probe does not make the health endpoint fail.
• Unit tests cover canonical counts, coherent snapshots and isolated probe failures.
• No catalogue contents are changed as part of this ticket.

Blocked by

None (can start immediately).