fix(backup): correct marker filenames in _validate_backup_zip

The backup validation checked for 'hermes_state.db' and 'memory_store.db'
as telltale markers of a valid Hermes backup zip. Neither name exists in a
real Hermes installation — the actual database file is 'state.db'
(hermes_state.py: DEFAULT_DB_PATH = get_hermes_home() / 'state.db').

A fresh Hermes installation produces:
  ~/.hermes/state.db        (actual name)
  ~/.hermes/config.yaml
  ~/.hermes/.env

Because the marker set never matched 'state.db', a backup zip containing
only 'state.db' plus 'config.yaml' would fail validation with:
  'zip does not appear to be a Hermes backup'
and the import would exit with sys.exit(1), silently rejecting a valid backup.

Fix: replace the wrong marker names with the correct filename.

Adds TestValidateBackupZip with three cases:
- state.db is accepted as a valid marker
- old wrong names (hermes_state.db, memory_store.db) alone are rejected
- config.yaml continues to pass (existing behaviour preserved)

hermes_cli/backup.py

@@ -201,7 +201,7 @@ def _validate_backup_zip(zf: zipfile.ZipFile) -> tuple[bool, str]:
         return False, "zip archive is empty"
 
     # Look for telltale files that a hermes home would have
-    markers = {"config.yaml", ".env", "hermes_state.db", "memory_store.db"}
+    markers = {"config.yaml", ".env", "state.db"}
     found = set()
     for n in names:
         # Could be at the root or one level deep (if someone zipped the directory)