docs: Archive all brainstorming docs

All hackathon-era brainstorming moved to archive/:
- 00_ROADMAP_SUMMARY.md (superseded by future-roadmap/)
- 01_PUBMED_IMPROVEMENTS.md (future work)
- 02_CLINICALTRIALS_IMPROVEMENTS.md (partially done, rest is future)
- 03_EUROPEPMC_IMPROVEMENTS.md (future work)
- BRAINSTORM_EMBEDDINGS_META.md (closed, decision made)
- UI_MODE_SELECTION_UX.md (obsolete, Simple mode removed)

Added README.md pointing to current docs.

docs/brainstorming/README.md

@@ -0,0 +1,22 @@
+# Brainstorming
+
+> **Status**: All brainstorming docs archived (December 2025)
+
+This folder contained early hackathon ideation. All documents have been moved to `archive/`.
+
+## Archived Documents
+
+| Document | Status | Notes |
+|----------|--------|-------|
+| `00_ROADMAP_SUMMARY.md` | Superseded | See `docs/future-roadmap/` |
+| `01_PUBMED_IMPROVEMENTS.md` | Future work | Rate limiting, full-text retrieval |
+| `02_CLINICALTRIALS_IMPROVEMENTS.md` | Partially done | Outcomes in SPEC-14, rest is future |
+| `03_EUROPEPMC_IMPROVEMENTS.md` | Future work | Full-text, citations |
+| `BRAINSTORM_EMBEDDINGS_META.md` | Closed | Decision: don't implement internal embeddings |
+| `UI_MODE_SELECTION_UX.md` | Obsolete | Simple mode removed, Anthropic removed |
+
+## Where to Look Now
+
+- **Future improvements**: `docs/future-roadmap/`
+- **Current architecture**: `docs/ARCHITECTURE.md`
+- **Active bugs**: `docs/bugs/ACTIVE_BUGS.md`