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/subscription.go

@@ -0,0 +1,64 @@
+package skillpack
+
+import "time"
+
+// Subscription is a host's persisted "I track this pack, pinned here" record. It
+// is metadata only — the pinned pack's bytes live in a PackCache keyed by
+// PinnedDigest. A subscription is only ever advanced to new content by an
+// explicit Apply (see Syncer): a sync records a PendingDigest, it never moves
+// the pin. That is the supply-chain guard — a compromised or careless upstream
+// cannot change what an agent runs without a human re-pin.
+type Subscription struct {
+	// ID is a stable host-assigned identifier.
+	ID string
+	// Name is the pack's manifest name (unique per host); what an agent lists in
+	// its SkillPacks and what skill_use receives.
+	Name string
+	// Description is the pinned manifest's description, cached so the catalog
+	// renders without opening the PackCache.
+	Description string
+
+	// Source coordinates.
+	SourceKind string // "dir" | "git"
+	SourceURL  string // dir path or git URL
+	Subpath    string // git subpath, if any
+	// TrackRef is the git commit-ish the host follows (branch/tag/SHA); empty =
+	// default branch. Sync fetches THIS; the pin only moves on Apply.
+	TrackRef string
+
+	// Pinned* describe the currently-active content.
+	PinnedDigest    string // content digest = PackCache key + change signal
+	PinnedSourceRef string // source's resolved ref (git SHA) — provenance
+	PinnedAt        time.Time
+	PinnedBy        string
+
+	// Pending* describe an update a sync found but has NOT applied. Empty
+	// PendingDigest = no pending update. A pending digest equal to the pinned
+	// one is impossible by construction (Syncer clears it).
+	PendingDigest    string
+	PendingSourceRef string
+	PendingAt        time.Time
+
+	// Enabled lets a host keep a subscription but deactivate it without
+	// deleting the pin/history.
+	Enabled bool
+}
+
+// HasPending reports whether a sync found an unapplied update.
+func (s *Subscription) HasPending() bool {
+	return s.PendingDigest != "" && s.PendingDigest != s.PinnedDigest
+}
+
+// pinTo advances the active pin to a fetched pack and clears any pending state.
+// Used by initial pin and by Apply.
+func (s *Subscription) pinTo(p *Pack, sourceRef, by string, now time.Time) {
+	s.Name = p.Manifest.Name
+	s.Description = p.Manifest.Description
+	s.PinnedDigest = p.Digest
+	s.PinnedSourceRef = sourceRef
+	s.PinnedAt = now
+	s.PinnedBy = by
+	s.PendingDigest = ""
+	s.PendingSourceRef = ""
+	s.PendingAt = time.Time{}
+}