chore: scaffold tools/foas library module

[yelizhenden-mdb]

What this introduces

This is Branch 1 of 4 in splitting tools/cli into a reusable library module (tools/foas) + a binary module (tools/cli). This branch is scaffolding only — it creates the empty-but-valid tools/foas module and wires up the workspace and CI. No packages are moved yet.

Files added

File	Purpose
tools/foas/go.mod	New module github.com/mongodb/openapi/tools/foas (Go 1.26, matching tools/cli)
tools/foas/doc.go	Root package foas doc stub — see note below
tools/foas/Makefile	build / unit-test / lint / fmt / deps / devtools (library variant — no binary build, e2e, or version injection)
tools/foas/.golangci.yml	Linter config, copied from tools/cli
tools/go.work	Workspace listing ./cli + ./foas so cli resolves foas locally during dev/CI
.github/workflows/code-health-foas-lib.yml	New CI: build + unit-test (ubuntu/macos matrix) + lint, triggered on tools/foas/**

Note on the doc.go stub

The design called for an "empty but valid module," but a truly empty module makes go test ./... exit non-zero (no packages to test), which would fail CI. A minimal root doc.go (package clause + doc comment, no logic) keeps the test step green. It'll sit alongside the real packages once they land in later branches.

Why foas (not foas-lib / foas-tools)

The module lives at github.com/mongodb/openapi/tools/foas. The tools/ parent already supplies the "tooling/library" grouping, and Go convention avoids hyphens in the final import-path element. The -lib/-cli suffixes are reserved for the workflow names, where they disambiguate two CI pipelines.

Verification

• tools/foas: go build ./... ✅ · go test ./... ✅ · go vet ./... ✅ · golangci-lint run → 0 issues ✅
• tools/cli: go build ./... ✅ under the workspace (unchanged)
• go work sync ✅ · workflow YAML valid ✅

Next branches

• Branch 2 — pilot: move apiversion into tools/foas, delete pkg/apiversion, update callers, add require + replace to cli/go.mod
• Branch 3 — move breakingchanges, changelog, openapi, pointer; delete pkg/ entirely
• Branch 4 — CI & release wiring (rename code-health-foascli.yml, add release-foas-lib.yml)