Fix skill bugs: session state init, agent field cleanup, /start path, /sprint-plan phases

- Remove invalid `agent: Explore` frontmatter from read-only skills (asset-audit, design-review, project-stage-detect, reverse-document)
- Fix design-system and map-systems to create session-state/active.md if it does not exist before updating
- Fix gate-check to remove reference to non-existent bmad-bmm-check skill
- Expand /start recommended paths into phased roadmap (Concept → Architecture → Production)
- Restructure /sprint-plan into numbered phases with clearer next-steps section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/skills/gate-check/SKILL.md

@@ -144,8 +144,8 @@ The project progresses through these stages:
 - [ ] Architecture document has no unresolved open questions in Foundation or Core layers
 - [ ] All ADRs have Engine Compatibility sections stamped with the engine version
 - [ ] All ADRs have ADR Dependencies sections (even if all fields are "None")
-- [ ] `/bmad-bmm-check-implementation-readiness` has been run or equivalent manual
-      validation confirms GDDs + architecture + epics are coherent
+- [ ] Manual validation confirms GDDs + architecture + epics are coherent
+      (run `/review-all-gdds` and `/architecture-review` if not done recently)
 - [ ] **Core fantasy is delivered** — at least one playtester independently described an experience that matches the Player Fantasy section of the core system GDDs (without being prompted).
 
 **Vertical Slice Validation** (FAIL if any item is NO):