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/memory.go

@@ -0,0 +1,114 @@
+package skillpack
+
+import (
+	"context"
+	"sort"
+	"sync"
+)
+
+// Memory is a zero-dependency in-process Store — a light host or a test gets
+// subscription persistence with no DB. Returned values are copies, so callers
+// can mutate them without corrupting the store.
+type Memory struct {
+	mu   sync.RWMutex
+	subs map[string]*Subscription // by ID
+}
+
+// NewMemory returns an empty in-memory Store.
+func NewMemory() *Memory {
+	return &Memory{subs: map[string]*Subscription{}}
+}
+
+var _ Store = (*Memory)(nil)
+
+func (m *Memory) Initialize(context.Context) error { return nil }
+
+func (m *Memory) Save(_ context.Context, s *Subscription) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	cp := *s
+	m.subs[s.ID] = &cp
+	return nil
+}
+
+func (m *Memory) Get(_ context.Context, id string) (*Subscription, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	s, ok := m.subs[id]
+	if !ok {
+		return nil, ErrNotFound
+	}
+	cp := *s
+	return &cp, nil
+}
+
+func (m *Memory) GetByName(_ context.Context, name string) (*Subscription, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	for _, s := range m.subs {
+		if s.Name == name {
+			cp := *s
+			return &cp, nil
+		}
+	}
+	return nil, ErrNotFound
+}
+
+func (m *Memory) List(context.Context) ([]Subscription, error) {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+	out := make([]Subscription, 0, len(m.subs))
+	for _, s := range m.subs {
+		out = append(out, *s)
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Name < out[j].Name })
+	return out, nil
+}
+
+func (m *Memory) Delete(_ context.Context, id string) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	delete(m.subs, id)
+	return nil
+}
+
+// MemoryPackCache is a zero-dependency in-process PackCache. Trees are copied on
+// the way in and out so a cached pin is immutable in practice.
+type MemoryPackCache struct {
+	mu    sync.RWMutex
+	trees map[string]Tree
+}
+
+// NewMemoryPackCache returns an empty in-memory PackCache.
+func NewMemoryPackCache() *MemoryPackCache {
+	return &MemoryPackCache{trees: map[string]Tree{}}
+}
+
+var _ PackCache = (*MemoryPackCache)(nil)
+
+func (c *MemoryPackCache) Put(_ context.Context, digest string, t Tree) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.trees[digest] = cloneTree(t)
+	return nil
+}
+
+func (c *MemoryPackCache) Get(_ context.Context, digest string) (Tree, error) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	t, ok := c.trees[digest]
+	if !ok {
+		return nil, ErrNotFound
+	}
+	return cloneTree(t), nil
+}
+
+func cloneTree(t Tree) Tree {
+	cp := make(Tree, len(t))
+	for k, v := range t {
+		b := make([]byte, len(v))
+		copy(b, v)
+		cp[k] = b
+	}
+	return cp
+}