ci: add first-class Go lane to repo-required-gate (go-ci.yml + go stack/polyglot)

[ArchonVII]

Problem

repo-required-gate supports stack: node | python | minimal | polyglot(node+python) but has no Go lane. A real consumer — ArchonVII/archon (10 tracked Go modules: root + tsunami + 8 tsunami/demo/*) — ships Go that the shared gate cannot build, vet, or test. Today archon routes **/*.go to language-ci, but the only language CI that runs is node-ci, which compiles zero Go. That is a false-green required check, not just a missing one.

The gate is now a source-of-truth abstraction, so Go must have an explicit state. This issue makes Go first-class.

Scope

1. New reusable .github/workflows/go-ci.yml mirroring node-ci.yml/python-ci.yml shape, but with multi-module discovery:
   • Discover module roots via git ls-files '**/go.mod' 'go.mod' (NOT a filesystem find — that catches gitignored build artifacts like dist/tsunamiscaffold/go.mod, and recurses into gitignored generated dirs such as tsunami/frontend/scaffold/dist/tw which contains conflicting package decls and breaks go build ./...). git ls-files returns exactly the tracked modules and excludes artifacts.
   • Per module: go mod download, go build ./..., go vet ./..., and go test ./... (test toggle).
   • Inputs: go-versions (JSON array), os-matrix, optional exclude-modules (newline globs), run-tests (bool), optional generate-command + verify-generated (run it, fail on dirty git diff — catches stale Go→TS bindings), cache toggle, build-tags/build-args passthrough (archon uses CGO/sqlite tags).
2. Classifier (scripts/classify-pr.mjs): add go to VALID_STACKS; emit runGoCi; generalize polyglot to a third language via go-paths (so archon = polyglot node+go). go.mod/go.sum already match DEFAULT_PACKAGE_RE and .go already matches DEFAULT_CODE_EXTENSIONS.
3. repo-required-gate.yml: add go-ci job + run-go-ci detect output + decision wiring + go-* inputs (go-versions, go-paths, go-test, go-generate-command, etc.).
4. Tests (scripts/classify-pr.test.mjs): new cases for stack: go, polyglot node+go routing, push events, forced-full; all 37 existing tests stay green.
5. Docs: new Go section in examples/repo-required-gate.yml; README workflow table; .changelog/unreleased/ fragment.

Verification

• npm test (vitest) green incl. new Go cases.
• actionlint clean on go-ci.yml + repo-required-gate.yml.

Release

After merge, v1 must be retagged (git tag -f v1 && git push -f origin v1) before consumers (archon) can use the Go lane.

Follow-up (not this issue)

languages: [<lang>] list-sugar on the caller + a resolver that asserts each declared language maps to a real lane — a cleaner surface than per-language *-paths, deferred to keep this PR reviewable.