cli QoL: vm prune, list→ls aliases, delete→rm aliases

- `banger vm prune` sweeps every non-running VM (stopped, created,
  error) with an interactive confirmation; -f/--force skips the prompt.
  Partial failures report which VM failed and exit non-zero.
- list commands gain `ls` alias: vm list already had it; added to image
  list, kernel list, and vm session list.
- delete commands gain `rm` alias: vm delete and image delete. kernel
  rm already aliased delete/remove.

Uses new test seams (vmListFunc) plus the existing vmDeleteFunc so
prune unit-tests without touching the daemon socket.

docs/advanced.md

@@ -17,6 +17,13 @@ banger vm stop testbox
 banger vm delete testbox
 ```
 
+Sweep every non-running VM (stopped, created, error) with:
+
+```bash
+banger vm prune          # interactive confirmation
+banger vm prune -f       # skip the prompt
+```
+
 `vm create` is synchronous by default, but on a TTY it shows live
 progress until the VM is fully ready.