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/model/types.go

@@ -144,7 +144,8 @@ type VMRecord struct {
 	LastTouchedAt time.Time `json:"last_touched_at"`
 	Spec          VMSpec    `json:"spec"`
 	Runtime       VMRuntime `json:"runtime"`
-	Stats         VMStats   `json:"stats"`
+	Stats         VMStats      `json:"stats"`
+	Workspace     VMWorkspace  `json:"workspace"`
 }
 
 type VMCreateRequest struct {
@@ -166,6 +167,18 @@ type VMSetRequest struct {
 	NATEnabled        *bool
 }
 
+// VMWorkspace records the last successful workspace.prepare result on
+// a VM so callers can skip re-stating the source path on every exec
+// and so banger can detect drift between the guest copy and the host
+// repo. Stored as workspace_json in the vms table; zero value means
+// no workspace has been prepared on this VM yet.
+type VMWorkspace struct {
+	GuestPath   string    `json:"guest_path,omitempty"`
+	SourcePath  string    `json:"source_path,omitempty"`
+	HeadCommit  string    `json:"head_commit,omitempty"`
+	PreparedAt  time.Time `json:"prepared_at,omitempty"`
+}
+
 type WorkspacePrepareMode string
 
 const (