feat: add /gsd:remove-phase command

Remove future phases from roadmap and renumber subsequent phases.
- Validates phase is unstarted (no SUMMARY.md files)
- Deletes phase directory
- Renumbers all subsequent phases (integers and decimals)
- Updates ROADMAP.md and STATE.md
- Git commit preserves historical record

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

README.md

@@ -307,6 +307,7 @@ You're never locked in. The system adapts.
 | `/gsd:new-milestone [name]`       | Create new milestone with phases                              |
 | `/gsd:add-phase`                  | Append phase to roadmap                                       |
 | `/gsd:insert-phase [N]`           | Insert urgent work                                            |
+| `/gsd:remove-phase [N]`           | Remove future phase, renumber subsequent                      |
 | `/gsd:discuss-phase [N]`          | Gather context before planning                                |
 | `/gsd:research-phase [N]`         | Deep ecosystem research for niche domains                     |
 | `/gsd:list-phase-assumptions [N]` | See what Claude thinks before you correct it                  |