Switch Spec

Switch to a different spec. The argument should be a spec ID.

Target: $ARGUMENTS

1. Validate the target first:
   • If no argument, show available specs and ask for target ID
   • If .specs/<target-id>/SPEC.md does not exist, report and stop
2. If target is already active, report "already active" and stop.
3. Pause current spec (if any) — run full pause workflow (save resume context, update checkboxes, set status to paused)
4. Load target spec — read .specs/<target-id>/SPEC.md
5. Activate it — set its status to active in both frontmatter and .specs/registry.md
6. Resume it — run the full resume workflow (parse progress, find current task, read resume context, present summary)
7. Update registry — ensure both specs' statuses are current in .specs/registry.md

This should feel like one seamless operation — context switch in a single command.