Exit as interrupted when Ctrl-C lands during a local index write (closes #301)

Once the run's context is cancelled, database/sql rolls an open
transaction back itself, so the rollback in WithTx returned
sql.ErrTxDone. WithTx treated any rollback error as fatal and called
os.Exit(1): an interrupted backup printed "FATAL: failed to rollback
transaction", exited 1 instead of 130 and skipped its deferred cleanup.

WithTx now ignores sql.ErrTxDone from the rollback and returns the
body's error. A rollback that really fails is returned together with
the body's error instead of ending the process. On a panic the
rollback error is dropped and the panic carries on.

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 Ctrl-C or SIGTERM during a local index write end the
+  command as interrupted
+  ([issue #301](https://git.eeqj.de/sneak/vaultik/issues/301)). Once the
+  run is cancelled, `database/sql` rolls an open transaction back itself,
+  so the rollback in `WithTx` failed with `sql.ErrTxDone` and the process
+  exited 1 with `FATAL: failed to rollback transaction`, skipping the
+  interrupted exit and message. That result is no longer an error, and a
+  rollback that does fail is returned with the transaction's error
+  instead of ending the process.
+
 - 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