Consolidate CLI verbs; retire overlapping commands

The verb surface accumulated overlapping cleanup commands. Consolidate
so each cleanup verb has one meaning:

- Rename 'database purge' -> 'database delete'. The command removes the
  SQLite file entirely; "purge" wrongly suggested purging contents.
- Fold 'snapshot cleanup' into 'prune'. Prune now runs three passes:
  reconcile local snapshots against the remote (previously the
  standalone cleanup command), drop orphaned local rows, then delete
  unreferenced remote blobs. One command, one mental model.
- Delete 'store info'. Its output was a strict subset of 'remote info',
  which already prints storage type + location. Any user reaching for
  either should reach for 'remote info'.
- Drop 'snapshot remove --all'. It duplicated 'remote nuke --force'.
  'remote nuke' is the single supported entry point for wiping the
  destination store.

Also update the storage-binding error message to reference the new
'vaultik database delete' name.

docs/DATAMODEL.md

@@ -7,7 +7,7 @@ Vaultik uses a local SQLite database to track file metadata, chunk mappings, and
 **Important Notes:**
 - **No Migration Support (pre-1.0)**: Vaultik does not support database schema
   migrations. The local index is treated as disposable — if the schema changes,
-  delete the local SQLite database (`vaultik database purge`) and run a full
+  delete the local SQLite database (`vaultik database delete`) and run a full
   backup. The remote storage is unaffected; the new index will re-deduplicate
   against existing remote blobs.
 - **Version Compatibility**: In rare cases, you may need to use the same version