docs: remove 11 outdated stage prompts and superseded docs

Deleted 8 historical STAGE*-SESSION-PROMPT.md files (all stages complete),
plus ai-session-brief.md, code-verification.md, and phase-roadmap.md
(superseded by CLAUDE.md and 254-test suite). Updated QUICK-REF.md
directory listing to reflect current docs structure.

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

docs/QUICK-REF.md

@@ -151,7 +151,9 @@ svsmspcalc/
     ├── SESSION-HANDOFF.md           # Current status + next steps
     ├── CHECKPOINT.md                # Historical checkpoint
     ├── MASTER-SESSION-PROMPT.md     # Master rebuild prompt
-    ├── STAGE3-11 prompts            # Historical stage prompts
+    ├── DECISION-LOG.md              # Decision history
+    ├── KNOWN-ISSUES.md              # Tracked bugs
     ├── quote-rules.md               # Business logic rules
-    └── regression-checklist.md      # QA checklist
+    ├── regression-checklist.md      # QA checklist
+    └── README.md                    # Project overview
 ```