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/cli/commands_vm.go

@@ -39,6 +39,7 @@ Quick reference:
   banger vm run ./repo -- make test      ship a repo, run a command, exit
   banger vm create --name dev            persistent VM; pair with 'vm ssh'
   banger vm ssh <name>                   open a shell in a running VM
+  banger vm exec <name> -- make test     run a command in the workspace with mise toolchain
   banger vm stop <name> | vm restart     graceful lifecycle
   banger vm kill <name>                  force-kill if stop hangs
   banger vm delete <name>                stop + remove disks
@@ -49,7 +50,7 @@ Quick reference:
 		Example: strings.TrimSpace(`
   banger vm run -- uname -a
   banger vm run ./project -- npm test
-  banger vm create --name agent && banger vm ssh agent
+  banger vm create --name dev && banger vm workspace prepare dev . && banger vm exec dev -- make test
 `),
 		RunE: helpNoArgs,
 	}
@@ -66,6 +67,7 @@ Quick reference:
 		d.newVMPruneCommand(),
 		d.newVMSetCommand(),
 		d.newVMSSHCommand(),
+		d.newVMExecCommand(),
 		d.newVMWorkspaceCommand(),
 		d.newVMLogsCommand(),
 		d.newVMStatsCommand(),