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>

docs/adr/20261023T214738672Z-adopt-a-checked-format-for-adrs-and-implementation-plans.md

@@ -1,8 +1,8 @@
 ---
 title: Adopt a checked format for ADRs and implementation plans
 date: 2026-10-23
-status: proposed
-implementation: in-progress
+status: accepted
+implementation: implemented
 summary: Use one documented ADR and plan format, enforced by a shared Go CLI (docs check).
 tags:
   - documentation
@@ -12,7 +12,8 @@ supersedes: []
 superseded_by: []
 related: []
 tracking: []
-implemented_by: []
+implemented_by:
+  - https://gitea.auvem.com/corestack/go/pulls/1
 ---
 
 # Adopt a checked format for ADRs and implementation plans