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>

cmd/docs/main.go

@@ -0,0 +1,66 @@
+// Command docs creates and checks ADR and plan records. See docs/docs-format.md.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"time"
+
+	"go.auvem.com/corestack/internal/docs"
+)
+
+const usage = `usage:
+  docs new adr|plan "Title"
+  docs check [--root DIR]
+`
+
+func main() { os.Exit(run(os.Args[1:], os.Stdout, os.Stderr, time.Now())) }
+
+// run returns the exit status: 0 success, 1 problems or failure, 2 usage error.
+func run(args []string, stdout, stderr io.Writer, now time.Time) int {
+	if len(args) == 0 {
+		fmt.Fprint(stderr, usage)
+		return 2
+	}
+	switch args[0] {
+	case "new":
+		if len(args) != 3 || (args[1] != "adr" && args[1] != "plan") {
+			fmt.Fprint(stderr, usage)
+			return 2
+		}
+		path, err := docs.New(".", args[1], args[2], now)
+		if err != nil {
+			fmt.Fprintf(stderr, "docs: %v\n", err)
+			return 1
+		}
+		fmt.Fprintln(stdout, path)
+		return 0
+	case "check":
+		fs := flag.NewFlagSet("docs check", flag.ContinueOnError)
+		fs.SetOutput(stderr)
+		root := fs.String("root", ".", "repository root")
+		if err := fs.Parse(args[1:]); err != nil || fs.NArg() != 0 {
+			if fs.NArg() != 0 {
+				fmt.Fprint(stderr, usage)
+			}
+			return 2
+		}
+		res, err := docs.Check(*root)
+		if err != nil {
+			fmt.Fprintf(stderr, "docs: %v\n", err)
+			return 1
+		}
+		for _, p := range res.Problems {
+			fmt.Fprintln(stdout, p)
+		}
+		if len(res.Problems) > 0 {
+			return 1
+		}
+		fmt.Fprintf(stdout, "ok: %d records checked\n", res.Records)
+		return 0
+	}
+	fmt.Fprint(stderr, usage)
+	return 2
+}