fix: address review findings on milestone-summary PR

1. Add <success_criteria> section to command (pattern compliance)
2. Fix archived audit path: check .planning/milestones/ not .planning/
3. Add RESEARCH.md to command context block
4. Add empty phase handling (graceful skip to minimal summary)
5. Add overwrite guard for existing summary files
6. Add 7 new tests: overwrite guard, empty phases, audit paths,
   success_criteria, RESEARCH.md context, artifact path resolution

Tests: 18/18 milestone-summary, 1184/1184 full suite

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

commands/gsd/milestone-summary.md

@@ -29,7 +29,7 @@ Output: MILESTONE_SUMMARY written to `.planning/reports/`, presented inline, opt
 - `.planning/RETROSPECTIVE.md`
 - `.planning/milestones/v{version}-ROADMAP.md` (if archived)
 - `.planning/milestones/v{version}-REQUIREMENTS.md` (if archived)
-- `.planning/phases/*-*/` (SUMMARY.md, VERIFICATION.md, CONTEXT.md)
+- `.planning/phases/*-*/` (SUMMARY.md, VERIFICATION.md, CONTEXT.md, RESEARCH.md)
 
 **User input:**
 - Version: $ARGUMENTS (optional — defaults to current/latest milestone)
@@ -38,3 +38,13 @@ Output: MILESTONE_SUMMARY written to `.planning/reports/`, presented inline, opt
 <process>
 Read and execute the milestone-summary workflow from @~/.claude/get-shit-done/workflows/milestone-summary.md end-to-end.
 </process>
+
+<success_criteria>
+- Milestone version resolved (from args, STATE.md, or archive scan)
+- All available artifacts read (ROADMAP, REQUIREMENTS, CONTEXT, SUMMARY, VERIFICATION, RESEARCH, RETROSPECTIVE)
+- Summary document written to `.planning/reports/MILESTONE_SUMMARY-v{version}.md`
+- All 7 sections generated (Overview, Architecture, Phases, Decisions, Requirements, Tech Debt, Getting Started)
+- Summary presented inline to user
+- Interactive Q&A offered
+- STATE.md updated
+</success_criteria>