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/errors.md

@@ -0,0 +1,21 @@
+# Errors and Result Serialization
+
+## Error taxonomy (`omnigraph::error::OmniError`)
+
+- `Compiler(...)` — schema/query parse/typecheck errors
+- `Lance(String)` — storage layer
+- `DataFusion(String)` — execution layer
+- `Io(io::Error)`
+- `Manifest(ManifestError { kind: BadRequest|NotFound|Conflict|Internal, … })`
+- `MergeConflicts(Vec<MergeConflict>)`
+
+Compiler-side `NanoError` covers parse / catalog / type / plan / execution / arrow / lance / IO / manifest / unique-constraint, each with structured spans (`SourceSpan { start, end }`) for ariadne-style diagnostics.
+
+## Result serialization (`omnigraph_compiler::result::QueryResult`)
+
+- `to_arrow_ipc()` — efficient binary
+- `to_sdk_json()` — JS-safe JSON (large i64 wrapped in metadata)
+- `to_rust_json()` — Rust-friendly JSON
+- `batches()` — direct Arrow `RecordBatch` access
+
+Mutation results: `{ affectedNodes: usize, affectedEdges: usize }` (also exposed as a tiny Arrow batch).