docs: update native-image CI guidance for Linux-only per-PR build

The per-PR native-image job no longer builds the macOS image and no longer
depends on the java/integration jobs, so the "macOS native image is the PR
long pole" guidance is stale.

Reflect the current setup: PRs build the Linux native image only (with
--quick-build-native, in parallel with the other jobs), while the macOS
native image is built out-of-band in native-image-macos.yml on push to
main, a weekly schedule, and workflow_dispatch.

Author: nielspardon

AGENTS.md

@@ -205,8 +205,11 @@ compile — they have their own visitor implementors:
   proto message usually needs: POJO + visitor wiring + both proto converters + a
   round-trip test, and often `ExpressionCreator` factories and `dsl/SubstraitBuilder`
   helpers for ergonomics.
-- When monitoring PR checks, budget for a long tail: the **macOS `Build Isthmus Native
-  Image`** job is the long pole — it `needs:` the `java` + `integration` jobs (so it
-  starts late), then AOT-compiles for ~15–20 min on a slower macOS runner. A PR staying
-  yellow after the other checks pass usually just means that job is still running, not
-  that it's stuck or failing.
+- When monitoring PR checks: the per-PR native-image job (`Build Isthmus Native Image` in
+  `pr.yml`) builds **Linux only**, uses `--quick-build-native`, and runs **in parallel**
+  with the `java` + `integration` jobs (no `needs:`), so it's fast and not a long pole. The
+  **macOS** native image is *not* built on PRs — it runs out-of-band in
+  `.github/workflows/native-image-macos.yml` (push to `main`, a weekly backstop, and
+  `workflow_dispatch`), fully optimized to mirror `release.yml`. So a PR that stays yellow
+  isn't waiting on a macOS native build, and macOS-specific native regressions surface on
+  `main` or the weekly run rather than on the PR.