chore: SparrowDB upgrade gate — export backup before swapping sparrowdb.node

[ryaker]

Problem

When sparrowdb.node is replaced (format change), the existing database becomes unreadable with no warning.
The upgrade should not happen until a backup exists AND the export/import tools are verified.

Solution

Document the upgrade procedure:

1. Run kms export → verify output file exists and node count matches
2. Swap sparrowdb.node
3. Run kms import <backup-file> into fresh DB path
4. Run kms_ping to verify counts match
5. Point SPARROWDB_PATH at new DB

Blocked by: export and import issues.