Refactor AGENTS.md from encyclopedia to map; move spec into docs/

Splits the 990-line AGENTS.md into a 184-line map (architecture,
where-to-find index, always-on invariants, capability matrix,
maintenance contract) plus 18 new docs/*.md files holding the deep
content per topic (storage, schema and query languages, indexes,
embeddings, branches/commits, runs, merge, changes, execution, policy,
server, CLI reference, audit, errors, CI, constants, v0.3.1 notes).

Adds scripts/check-agents-md.sh and a check_agents_md CI job that
verifies every docs/ link in AGENTS.md resolves and every doc in the
canonical set is linked. CLAUDE.md remains a symlink to AGENTS.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -99,6 +99,18 @@ jobs:
           echo "run_full_ci=$run_full_ci" >> "$GITHUB_OUTPUT"
           echo "run_rustfs_ci=$run_rustfs_ci" >> "$GITHUB_OUTPUT"
 
+  check_agents_md:
+    name: Check AGENTS.md Links
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Checkout source
+        uses: actions/checkout@v5.0.1
+
+      - name: Verify AGENTS.md ↔ docs/ cross-links
+        run: bash scripts/check-agents-md.sh
+
   test:
     name: Test Workspace
     needs: classify_changes