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
