speckit-bugfix-switchlisted

# Switch bugfix context Update current context into existing feature. The feature identity is define by user. ## User Input ```text $ARGUMENTS ``` ## Outline 1. **Validate feature**: - Check `specs/{feature}` dir exists, report an error to user if missing. 2. **Checkout feature branch**: - Checkout a new `{feature}` branch using git command, always use main branch as source. - If current branch contains uncommitted changes STOP immediately and escalate to project maintainers for guidance. 3. **Feature context update**: - Update `.specify/feature.json` with `specs/{feature}` 4. **Agent context update**: - Update the plan reference between the `<!-- SPECKIT START -->` and `<!-- SPECKIT END -->` markers in `.github/copilot-instructions.md` to point to the feature plan file `specs/{feature}/plan.md`