test(go-ci): add workflow-level self-test for the Go lane

[ArchonVII]

Summary

go-ci.yml (added in #52) shipped with no workflow-level execution test — self-test.yml only runs vitest over the classifier scripts and never invokes any reusable lane. So the Go lane's 200-line shell (discovery, go build/go vet/go test, exclude-modules) had never actually run in Actions.

This adds self-test-go.yml, which calls go-ci.yml through a local (./) reusable ref — so the in-PR version is the one under test — against committed fixture modules under test/fixtures/go/:

• alpha/ — valid module with a passing test → proves go build + go vet + go test all execute.
• beta/ — second valid module → proves multi-module discovery (git ls-files finds more than one go.mod).
• excluded/ — intentionally non-compiling, passed via exclude-modules → a green run self-validates that exclusion actually skips it. If exclusion ever regresses, the broken module builds and the job fails loudly.

cache: false because the dependency-less fixtures have no go.sum, and actions/setup-go with caching errors when **/go.sum matches nothing. Caching is actions/setup-go behavior, not what this self-test proves.

Why now

This closes the self-test gap #52 shipped with and lets the v1 retag be proven, not hopeful. Sequence: merge this → retag v1 → archon #145 Phase 3 consumes the Go lane via @v1.

Verification

Local (Windows, Go 1.26.3), with fixtures staged so git ls-files discovery is realistic:

discovery -> test/fixtures/go/{alpha,beta,excluded}
alpha:    go build/vet/test -> ok  example.com/selftest/alpha
beta:     go build/vet      -> ok  (no test files, exit 0)
excluded: go build          -> FAIL: undefined: undefinedOnPurpose (exit 1)  # proves exclusion is a real assertion

• actionlint v1.7.12 on self-test-go.yml + go-ci.yml → clean.
• The authoritative build/vet/test on the ubuntu lane is this PR's own Self-test (go lane) check.

Scope boundary

This proves discovery + build/vet/test + exclude-modules. The generate-command/verify-generated drift path (trivial git status --porcelain shell) is left unexercised here; archon #145 Phase 3 is its real-world consumer and will prove it in that PR's CI.

Docs / Changelog

.changelog/unreleased/55-go-ci-selftest.md added. Fixtures documented in test/fixtures/go/README.md.

Linked Issue

Closes #55