Make snapshot remove fail for an unknown snapshot ID (closes #293)

snapshot remove never checked that the ID existed: it deleted nothing,
reported the snapshot removed from the local database and from the
destination store, and exited 0, under --json, --dry-run and
--local-only too. It now looks the ID up in the local index and, only
when that has no row and --local-only is not set, on the destination
store; finding neither is an error. The command reports a removal from
the local database only when the local index had the row, and
remote_removed is true only when metadata was actually deleted.

Judgement call: under --local-only the destination store is not
consulted, so the snapshot must be in the local index.

Model: opus-5-5

TODO.md

@@ -22,6 +22,16 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-13: Made `snapshot remove` fail for a snapshot that does not
+  exist ([issue #293](https://git.eeqj.de/sneak/vaultik/issues/293)).
+  The command never checked the ID: it reported the snapshot removed
+  from the local database and from the destination store and exited 0,
+  under `--json`, `--dry-run` and `--local-only` too. An ID that names
+  no snapshot in the local index or on the destination store is now an
+  error; with `--local-only`, the snapshot must be in the local index.
+  The command now reports a removal from either side only when one
+  happened, and `remote_removed` is true only when metadata was deleted.
+
 - 2026-10-12: Rejected a retention duration longer than about 292 years
   ([issue #289](https://git.eeqj.de/sneak/vaultik/issues/289)). The
   number given to `--older-than` or `--keep-newer-than` was multiplied