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/store.go

@@ -0,0 +1,27 @@
+package skillpack
+
+import "context"
+
+// Store is the persistence seam for subscriptions (metadata + the current pin).
+// It is deliberately small; a host backs it with its DB, Memory is the zero-dep
+// default, and contrib/store can add durable SQLite alongside the other
+// executus store impls.
+type Store interface {
+	Initialize(ctx context.Context) error
+	Save(ctx context.Context, s *Subscription) error
+	Get(ctx context.Context, id string) (*Subscription, error)
+	GetByName(ctx context.Context, name string) (*Subscription, error)
+	List(ctx context.Context) ([]Subscription, error)
+	Delete(ctx context.Context, id string) error
+}
+
+// PackCache is the content-addressed store of pinned pack trees, keyed by
+// content digest. It exists so activating an agent never re-fetches from the
+// Source (no clone per run) and so a pinned digest's exact bytes survive even if
+// upstream later force-pushes or disappears. A host may back it with disk;
+// Memory is the default. Because the key IS the content digest, entries are
+// immutable and safe to share across subscriptions that pin the same bytes.
+type PackCache interface {
+	Put(ctx context.Context, digest string, t Tree) error
+	Get(ctx context.Context, digest string) (Tree, error)
+}