smoke: five more scenarios + fix exit-code propagation bug the new ones caught

Five new smoke scenarios layered on top of the existing bare + workspace
vm-run tests:

  - exit-code propagation: `sh -c 'exit 42'` must rc=42
  - workspace dry-run: --dry-run lists tracked files without a VM
  - workspace --include-untracked: opt-in ships files outside the git
    index (regression guard on the security-default flip from review 4)
  - concurrent vm runs: two --rm invocations in parallel both succeed
    (stresses per-VM locks, createVMMu reservation window, tap pool)
  - invalid spec rejection: --vcpu 0 must fail with no VM row left
    behind (the "cleanup on partial failure" path the review flagged)

The exit-code scenario caught a real bug on first run:

  `banger vm run --rm -- sh -c 'exit 42'` returned rc=0, not 42.

Root cause in internal/cli/ssh.go's sshCommandArgs: extra args were
appended to the ssh argv verbatim, relying on ssh(1)'s implicit
space-join to deliver the remote command. That works for single
tokens (echo hello) but re-tokenises multi-word commands on the
remote side: `ssh host sh -c 'exit 42'` becomes remote
`sh -c exit 42`, where `42` is $0 for the already-completed `exit`,
and the exit code the user asked for is lost.

Fix: shell-quote every element of extra (`'sh'` `'-c'` `'exit 42'`)
and join them into a single trailing argv entry. ssh's space-join
then produces exactly the command the user typed on the remote
shell. TestSSHCommandArgs was updated to pin the quoting; the
existing TestRunVMRunCommandModePropagatesExitCode test needed a
one-word quote tweak (`false` → `'false'`).

Smoke run after fix passes all seven scenarios in ~2 min on warm
state. cmd/banger coverage jumped to 100% (the invalid-spec
scenario hits the error-reporting path that wasn't covered
before).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/cli/ssh.go

@@ -91,7 +91,20 @@ func sshCommandArgs(cfg model.DaemonConfig, guestIP string, extra []string) ([]s
 		)
 	}
 	args = append(args, "root@"+guestIP)
-	args = append(args, extra...)
+	// ssh(1) concatenates every argument after the host with spaces
+	// before sending to the remote shell. That means passing extra
+	// args raw — `ssh host sh -c 'exit 42'` — re-tokenises on the
+	// remote side to `sh -c exit 42`, where `42` is $0 for the
+	// already-completed `exit`, and the rc the user asked for is
+	// lost. Shell-quote each element and join them ourselves so the
+	// remote shell sees exactly the argv the user typed locally.
+	if len(extra) > 0 {
+		quoted := make([]string, len(extra))
+		for i, a := range extra {
+			quoted[i] = shellQuote(a)
+		}
+		args = append(args, strings.Join(quoted, " "))
+	}
 	return args, nil
 }