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

[ArchonVII]

Summary

Adds a first-class Go lane to the shared CI surface. repo-required-gate supported node/python/minimal/polyglot but had no way to build, vet, or test Go — a real consumer (ArchonVII/archon, 10 tracked Go modules) routed .go changes to language-ci while only node-ci ran (a false-green required check).

• New reusable go-ci.yml — discovers every tracked go.mod via git ls-files (deliberately NOT a filesystem find: that catches gitignored build artifacts like dist/<scaffold>/go.mod and recurses into generated dirs whose conflicting package decls break go build ./...). Runs go build / go vet / go test per module. Optional setup-command, generate-command + verify-generated (dirty-tree drift check for committed codegen), exclude-modules, cgo-enabled, and build/test arg passthrough.
• Classifier — stack: go + runGoCi; polyglot generalized to a third language via go-paths; lane label lists active languages (e.g. code (polyglot: node+go)).
• Gate — go-* inputs, run-go-ci detect output, a go-ci job, and decision wiring.
• Docs — standalone examples/go-ci.yml, a node+go polyglot example, a README row, and a changelog fragment.

Concrete Go support first; the languages: [<lang>] list-sugar + resolver refinement is deferred to a follow-up to keep this reviewable.

Verification

• Classifier unit tests (TDD: 12 new, written failing first, then implemented)
• actionlint on all changed/new workflows
• Discovery + exclusion logic validated against the real archon tree

Verification Notes

• npx vitest run → 116 passed (6 files), including 12 new Go cases (stack=go, polyglot node+go, node+python+go label ordering, push + forced-full events, all-empty-paths guardrail).
• actionlint go-ci.yml repo-required-gate.yml examples/go-ci.yml examples/repo-required-gate.yml → exit 0.
• Ran go-ci.yml's discovery + exclude-modules shell against C:\GitHub\archon: found all 10 tracked modules (root + tsunami + 8 tsunami/demo/*); exclude-modules: tsunami/demo/ correctly kept root + tsunami and skipped the 8 demos. (root + tsunami also go build/go vet clean on archon main.)
• shellcheck is not installed locally; go-ci.yml's run: blocks carry # shellcheck disable=SC2086 for the two intentional word-split arg expansions.

Docs / Changelog

.changelog/unreleased/51-go-ci-lane.md added; README Generic-CI table updated; examples added/updated.

Linked Issue

Closes #51

Release note

Consumers (e.g. archon) can use the Go lane only after v1 is retagged (git tag -f v1 && git push -f origin v1) post-merge.