Add pre-commit hook for openapi.json drift

Wire a local pre-commit hook that regenerates openapi.json whenever
the server source changes, and document the workflow in CONTRIBUTING.
Opt-in via `pre-commit install`; the existing CI test remains the
authoritative drift check.

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

CONTRIBUTING.md

@@ -16,6 +16,24 @@ cargo test --workspace
 If you touch S3-backed flows, the CI model uses a local RustFS instance for
 integration tests.
 
+### OpenAPI spec
+
+`openapi.json` is a committed artifact generated from the Utoipa annotations in
+`crates/omnigraph-server`. CI fails if it drifts from the source. To regenerate
+manually:
+
+```bash
+OMNIGRAPH_UPDATE_OPENAPI=1 cargo test -p omnigraph-server --test openapi openapi_spec_is_up_to_date
+```
+
+Optional: install [pre-commit](https://pre-commit.com) to run this automatically
+before each commit that touches the server:
+
+```bash
+pip install pre-commit
+pre-commit install
+```
+
 ## Pull Requests
 
 - keep changes focused