feat(vm): add vm exec command with workspace dirty detection

Introduces three interconnected features for persistent VM workflows:

1. `banger vm exec <vm> -- <cmd>`: runs a command in the prepared
   workspace, automatically cd-ing into the guest path and wrapping
   via `mise exec --` so mise-managed tools are on PATH. Falls back
   to a plain exec when mise isn't available. Exit code propagates
   verbatim.

2. Workspace persistence: workspace.prepare now stores the guest path,
   host source path, and HEAD commit into a new `workspace_json` column
   on the vms table (migration 3). This state survives daemon restarts
   and informs both dirty-checking and auto-prepare.

3. Dirty detection: `vm exec` compares the stored HEAD commit against
   the current host repo HEAD. When stale it warns and, with
   --auto-prepare, re-syncs the workspace before running.

Also:
- WORKSPACE column added to `banger ps` / `vm list`
- `banger vm` quick reference updated with `vm exec` entry

internal/daemon/workspace_test.go

@@ -619,7 +619,7 @@ func TestMiseTrustScriptShape(t *testing.T) {
 	got := miseTrustScript("/root/repo")
 	for _, want := range []string{
 		"command -v mise",
-		"mise trust --quiet --all '/root/repo'",
+		"cd '/root/repo' && mise trust --quiet --all",
 		"|| true",
 	} {
 		if !strings.Contains(got, want) {