feat: add docs checker for ADRs and implementation plans (#1)

Add cmd/docs, the first corestack component: `docs new adr|plan "Title"`
writes records from embedded templates, and `docs check` validates them
against the public format spec in docs/docs-format.md (timestamp IDs,
frontmatter keys and enums, separate decision and implementation status,
supersession symmetry, MADR 4 layout with required Consequences, plan
sections, and repo-root/durable references), reporting path:line errors.

Also adds the module skeleton (go.auvem.com/corestack, BSD-3-Clause,
README, AGENTS.md, CLAUDE.md), Gitea CI running tests and docs check,
a tag-triggered goreleaser release for darwin/linux binaries, and the
accepted ADR adopting the format.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

go.mod

@@ -0,0 +1,8 @@
+module go.auvem.com/corestack
+
+go 1.27.1
+
+require (
+	github.com/yuin/goldmark v1.8.6
+	go.yaml.in/yaml/v3 v3.0.5
+)