docs: record how CI is set up, and what broke it

Two jobs, which runners serve them, and the two constraints that kept
the x86 job failing on every push until today: no JavaScript actions
(`rust:latest` has no `node`, and GitHub is not reachable from every
runner) and `cmake` for libz-ng-sys. Also notes that the Docker Hub
`latest` tag for the runner is frozen at 0.6.1, so it is not a way to
stay current.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

CLAUDE.md

@@ -116,6 +116,23 @@ cargo build --release
 cargo test --workspace
 ```
 
+### CI
+`.gitea/workflows/ci.yml` has two jobs, both green as of 2026-09-22:
+- **`test`** (`ubuntu-latest`, in `rust:latest`) runs `scripts/ci-test.sh` with
+  the h5py/netCDF4 interop suites required (`CLAWHDF5_REQUIRE_INTEROP=1`).
+  Served by the `tank` and `architect` runners.
+- **`test-arm64`** (`linux_arm64`) lints and tests the aarch64 code — the NEON
+  kernels are `cfg`'d out on x86, so this is the only place they are built.
+  Served by `vision-01` (host mode) and `vision-02` (Docker), so steps must
+  work in both.
+
+Keep workflows free of JavaScript actions (`actions/checkout`, `actions/cache`,
+…): `rust:latest` has no `node`, and not every runner reaches GitHub, where
+they are fetched from. Check out with plain `git` instead. Both jobs need
+`cmake` for `libz-ng-sys` (from `clawhdf5-format`'s default `fast-deflate`).
+All runners are on `gitea-runner` 3.5.0, from `docker.gitea.com/act_runner`
+— `gitea/act_runner:latest` on Docker Hub is frozen at 0.6.1.
+
 ### CLI
 ```bash
 cargo run -p clawhdf5-cli -- --help