ci(tests): run the Oh My Pi compatibility suite as a required check

tests/test-omp-compat.mjs skipped whenever omp was not on PATH, which
was always the case in CI, so 0.6.1 shipped an import that pi's compat
entrypoint provides but OMP's bundled pi-ai does not (#74).

Add an omp-compat job that installs @oh-my-pi/pi-coding-agent with Bun
and runs the suite with OMP_COMPAT_REQUIRED=1, which turns the skip into
a failure. Add a first phase that loads the extension through
`omp models -e`, the same loader `omp plugin install` validates with;
it fails on the 0.6.1 index.ts and passes with the runtime resolution.

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Fix `omp plugin install` on Oh My Pi 18.x, which rejected 0.6.1 because its pi-ai lacks the `registerApiProvider` export; the compat registration now resolves at runtime and is skipped on hosts that register custom APIs themselves.
+- Run the Oh My Pi compatibility suite against a real `omp` binary in CI as a required check, and assert there that the extension loads against OMP's bundled pi packages.
 
 ## 0.6.1 - 2026-09-01