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>

docs/changes.md

@@ -0,0 +1,24 @@
+# Change Detection / Diff
+
+`changes/mod.rs`. Three-level algorithm:
+
+1. **Manifest diff**: skip sub-tables whose `(table_version, table_branch)` is unchanged.
+2. **Lineage check**:
+   - Same branch lineage → fast path: use the per-row `_row_last_updated_at_version` column to classify Insert/Update/Delete.
+   - Different lineages → ID-based streaming comparison.
+3. **Row-level diff**: streaming, no full materialization.
+
+## Public API
+
+- `diff_between(from: ReadTarget, to: ReadTarget, filter: Option<ChangeFilter>) -> ChangeSet`
+- `diff_commits(from_commit_id, to_commit_id, filter)` — cross-branch safe.
+
+## Types
+
+```
+ChangeOp: Insert | Update | Delete
+EntityKind: Node | Edge
+EntityChange { table_key, kind, type_name, id, op, manifest_version, endpoints?: {src, dst} }
+ChangeFilter { kinds?, type_names?, ops? }
+ChangeSet { from_version, to_version, branch?, changes[], stats }
+```