fix: /mgw:project creates STATE.md and archives old phases

[snipcodeit]

Summary

• Adds archive_phases step to /mgw:project that moves existing .planning/phases/ to .planning/ARCHIVED-phases-pre-{name}-{date}/ before writing a new ROADMAP
• Adds write_state step that creates .planning/STATE.md from the GSD template, initialized for Phase 1 with skip-if-exists guard
• Updates the report and success_criteria sections to reflect both new steps

Closes #33

Changes

commands/project.md (+230 lines):

• New archive_phases step inserted before write_roadmap — checks for existing phase directories, archives them with a timestamped name, sets ARCHIVE_STATUS
• New write_state step inserted after write_roadmap — creates full GSD-compliant STATE.md with all 8 sections (Project Reference, Current Position, Performance Metrics, Accumulated Context, Session Continuity), sets STATE_STATUS
• Updated report step GSD scaffold section to display ROADMAP_STATUS, STATE_STATUS, and ARCHIVE_STATUS
• Updated success_criteria with two new checklist items

.claude/commands/mgw/project.md (mirror):

• Byte-identical copy of commands/project.md

Test Plan

• Verify archive_phases step appears before write_roadmap in commands/project.md
• Verify write_state step appears after write_roadmap and before write_project_json
• Verify STATE.md template follows GSD template format (all 8 sections present)
• Verify both files are byte-identical: diff commands/project.md .claude/commands/mgw/project.md
• Verify report section shows all three status variables
• Verify success_criteria includes STATE.md and archival checks
• Run /mgw:project on a test repo with existing .planning/phases/ — confirm archival + STATE.md creation
• Run /mgw:project on a clean repo — confirm STATE.md created, archival skipped gracefully

Verification

GSD verifier passed 5/5 must-haves:

1. archive_phases step exists before write_roadmap
2. write_state step exists after write_roadmap
3. STATE.md template is GSD-compliant
4. Report and success_criteria updated
5. Mirror files byte-identical

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Testing Procedures

Quick Verification (code review)

# Step ordering
grep -n 'step name="archive_phases"\|step name="write_roadmap"\|step name="write_state"\|step name="write_project_json"' commands/project.md

# Mirror identical
diff commands/project.md .claude/commands/mgw/project.md

# STATE.md template sections present
grep -c '## Project Reference\|## Current Position\|## Performance Metrics\|## Accumulated Context\|## Session Continuity' commands/project.md

End-to-End Test

1. Create a temp repo with existing .planning/phases/01-test/ directory
2. Run /mgw:project against it
3. Verify:
   • .planning/phases/ is gone, replaced by .planning/ARCHIVED-phases-pre-{name}-{date}/
   • .planning/STATE.md exists with Phase 1 initialization
   • /gsd:health reports HEALTHY (no E004)
4. Run /mgw:project again (idempotent check):
   • STATE.md should be skipped ("exists")
   • ROADMAP.md should be skipped ("exists")

Edge Cases

• No existing .planning/phases/ → archival skipped gracefully
• STATE.md already exists → write_state skipped
• Empty .planning/phases/ directory → archival skipped

[snipcodeit]

Closing — wrong approach. This PR adds more GSD file management to MGW, but the correct fix is the opposite: MGW should stop writing .planning/ROADMAP.md entirely and let GSD manage all .planning/ files. See replacement issue for the correct architecture.