Release v0.20.0: /docs/specs surface + nested flyout nav + session body-list removal

Wave 9 follow-up to roadmap item #30 (Session 0017.0 shipped #30 as v0.19.0;
v0.20.0 lands the operator-feedback follow-ups on top).

1. Specs on /docs/specs/<name> (backend docs_specs.py + frontend DocsSpec.jsx
   + DocsSpecsIndex.jsx). Configured via OHM_DOCS_SPECS; framework default
   carries OHM's two specs (rfc-app/SPEC.md + flotilla SPEC.md). Runtime
   fetch from gitea raw with 5-min TTL cache, mirroring docs_sessions.py.

2. Nested flyout nav hierarchy (DocsLayout.jsx). Sessions render as a tree
   with transcripts nested under each session row (labeled by .N ordinal).
   New Specs section between User Guide and Sessions.

3. /docs/sessions/<NNNN> body-list removed (DocsSessionIndex.jsx). Body
   becomes a session-overview card; navigation lives in the left nav.

19 new pytest cases for docs_specs (332 backend total green). Frontend
build clean. Sync frontend/package-lock.json version drift (0.15.0 → 0.20.0)
alongside the VERSION + package.json bump.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "rfc-app-frontend",
   "private": true,
-  "version": "0.19.0",
+  "version": "0.20.0",
   "type": "module",
   "scripts": {
     "dev": "vite",