feat(skillpack): SKILL.md-subscription battery

New additive, nil-safe battery for subscribing to skill packages in the
Anthropic agent-skills format (SKILL.md manifest + bundled files):

- Manifest/ParseManifest: SKILL.md frontmatter+body parse & validation
  (name/description required, allowed-tools passthrough, kebab/length limits)
- Tree/Pack/LoadPack: self-contained file set, order-independent content
  digest (the pin identity + change signal), bundled-file listing, traversal-
  safe staging
- Source (DirSource, GitSource): Fetch returns tree + resolved ref; git clones
  to temp, reads subpath into memory, cleans up (self-contained tree)
- Subscription + Store + content-addressed PackCache, with Memory defaults
- Syncer: Subscribe pins; Check records a PENDING update but never moves the
  pin; Apply is the only re-pin (supply-chain guard — upstream can't silently
  change what an agent runs)
- Activate: resolved packs -> majordomo agent.Skill (catalog instructions +
  one skill_use tool) for progressive disclosure; Stage materializes files

Third distinct 'skill' concept, deliberately separate from executus/skill
(saved-agent noun) and majordomo/skill (eager capability bundle). Mort-side
wiring (convars, .skillpack commands, Agent.SkillPacks, allowed-tools shim)
is a later, separate step. Full unit + hermetic local-git tests; gofmt/vet
clean; race-tested.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

skillpack/pack_test.go

@@ -0,0 +1,80 @@
+package skillpack
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func sampleTree() Tree {
+	return Tree{
+		ManifestName:         []byte(goodManifest),
+		"scripts/fill.py":    []byte("print('hi')\n"),
+		"references/spec.md": []byte("# spec\n"),
+	}
+}
+
+func TestTreeDigest_StableAndContentSensitive(t *testing.T) {
+	a := sampleTree()
+	b := sampleTree()
+	if a.Digest() != b.Digest() {
+		t.Fatal("identical trees must share a digest")
+	}
+	b["scripts/fill.py"] = []byte("print('bye')\n")
+	if a.Digest() == b.Digest() {
+		t.Fatal("content change must change the digest")
+	}
+	// Adding a file changes the digest.
+	c := sampleTree()
+	c["extra.txt"] = []byte("x")
+	if a.Digest() == c.Digest() {
+		t.Fatal("added file must change the digest")
+	}
+}
+
+func TestLoadPack(t *testing.T) {
+	p, err := LoadPack(sampleTree())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p.Manifest.Name != "pdf-processing" {
+		t.Errorf("name = %q", p.Manifest.Name)
+	}
+	if len(p.Bundled) != 2 || p.Bundled[0] != "references/spec.md" || p.Bundled[1] != "scripts/fill.py" {
+		t.Errorf("bundled = %v (want sorted, sans SKILL.md)", p.Bundled)
+	}
+	if p.Digest == "" {
+		t.Error("digest empty")
+	}
+}
+
+func TestLoadPack_NoManifest(t *testing.T) {
+	if _, err := LoadPack(Tree{"readme.md": []byte("x")}); err != ErrNoManifest {
+		t.Fatalf("want ErrNoManifest, got %v", err)
+	}
+}
+
+func TestTreeWriteTo(t *testing.T) {
+	dir := t.TempDir()
+	if err := sampleTree().WriteTo(dir); err != nil {
+		t.Fatal(err)
+	}
+	got, err := os.ReadFile(filepath.Join(dir, "scripts", "fill.py"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "print('hi')\n" {
+		t.Errorf("staged content = %q", got)
+	}
+}
+
+func TestTreeWriteTo_RejectsTraversal(t *testing.T) {
+	dir := t.TempDir()
+	evil := Tree{"../escape.txt": []byte("nope")}
+	if err := evil.WriteTo(dir); err == nil {
+		t.Fatal("expected traversal rejection")
+	}
+	if _, err := os.Stat(filepath.Join(filepath.Dir(dir), "escape.txt")); err == nil {
+		t.Fatal("traversal file was written outside dir")
+	}
+}