refactor: remove dead code, improve execute-phase UX, fix requirements flow

- Remove phantom status.md command (background agent model abandoned)
- Remove agent-history.md template (unused)
- Remove _archive/ directory
- Add narration to execute-phase (describe what's being built before/after waves)
- Update new-project to offer define-requirements as fast path
- Make define-requirements work without research (gather through questioning)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -340,7 +340,6 @@ You're never locked in. The system adapts.
 | `/gsd:plan-phase [N]` | Generate task plans for phase |
 | `/gsd:execute-plan` | Run single plan via subagent |
 | `/gsd:execute-phase <N>` | Execute all plans in phase N with parallel agents |
-| `/gsd:status [--wait]` | Check background agent status from parallel execution |
 | `/gsd:progress` | Where am I? What's next? |
 | `/gsd:verify-work [N]` | User acceptance test of phase or plan ¹ |
 | `/gsd:plan-fix [plan]` | Plan fixes for UAT issues from verify-work |