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>

AGENTS.md

@@ -0,0 +1,9 @@
+# AGENTS.md
+
+Go module `go.auvem.com/corestack`. Reading map:
+
+- Build and test: `go build ./...`, `go vet ./...`, `go test -race ./...`; format with `gofmt -w .`.
+- Docs format (the spec `docs check` implements): `docs/docs-format.md`.
+- Check records: `go run ./cmd/docs check`. Create one: `go run ./cmd/docs new adr|plan "Title"`.
+- Decisions live in `docs/adr/`, plans in `docs/plans/`. Records are permanent; never rename them.
+- Dependencies are limited to `go.yaml.in/yaml/v3` and `github.com/yuin/goldmark`.