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>

.goreleaser.yaml

@@ -0,0 +1,29 @@
+version: 2
+project_name: docs
+
+builds:
+  - id: docs
+    main: ./cmd/docs
+    binary: docs
+    env:
+      - CGO_ENABLED=0
+    goos: [darwin, linux]
+    goarch: [amd64, arm64]
+    flags: [-trimpath]
+    ldflags: [-s -w]
+
+archives:
+  - formats: [tar.gz]
+    name_template: "docs_{{ .Version }}_{{ .Os }}_{{ .Arch }}"
+
+checksum:
+  name_template: checksums.txt
+
+gitea_urls:
+  api: https://gitea.auvem.com/api/v1
+  download: https://gitea.auvem.com
+
+release:
+  gitea:
+    owner: corestack
+    name: go