Exit 130 and say so when a command is interrupted (closes #267)

Ctrl-C or SIGTERM during snapshot create, restore or verify exited 0
with no error line (1, also silent, under snapshot verify --json), so
an unfinished --cron backup looked like a success. RunOperation now
counts an op as interrupted when the Vaultik context was cancelled
before it returned, rather than when its error wraps context.Canceled,
which verify --json does not. Entry prints "interrupted before the
command finished" on stderr for it and returns 130, under --cron and
--json too.

SIGTERM also gives 130, as the issue asks, not 143.
The test does not reach the branch for an op still running when the
30s shutdown timeout ends.

Model: opus-5-5

TODO.md

@@ -22,6 +22,15 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Made an interrupted command exit 130 and say so
+  ([issue #267](https://git.eeqj.de/sneak/vaultik/issues/267)). Ctrl-C
+  or SIGTERM during `snapshot create`, `snapshot restore` or `snapshot
+  verify` exited 0 with no error line (`snapshot verify --json` exited
+  1, also without one), so a `--cron` run that never finished looked
+  like a success. A command stopped by either signal now exits 130 and
+  prints `interrupted before the command finished` on stderr, under
+  `--cron` and `--json` too.
+
 - 2026-10-07: Corrected documentation, help text and comments that were
   false about the code
   ([issue #233](https://git.eeqj.de/sneak/vaultik/issues/233)). A blob