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/skillpack.go

@@ -0,0 +1,54 @@
+// Package skillpack is the SKILL.md-subscription battery: it lets an agent host
+// subscribe to skill packages published as directories/git repos in the
+// Anthropic "agent skills" format (a SKILL.md manifest plus optional bundled
+// scripts and reference files) and activate them for a run with progressive
+// disclosure.
+//
+// It is a THIRD, distinct concept from the two "skill" nouns already in the
+// stack — do not conflate them:
+//
+//   - majordomo/skill  — a lightweight capability bundle (instructions + tools)
+//     appended to an agent eagerly at construction.
+//   - executus/skill   — a heavyweight persisted "saved agent" noun.
+//   - executus/skillpack (this package) — an externally-authored, versioned,
+//     on-demand-loaded instruction pack fetched from a Source and pinned by
+//     content digest.
+//
+// Progressive disclosure is the reason this is not just a majordomo/skill:
+// majordomo skills inject their whole instruction text into the system prompt
+// up front, which does not scale to a catalog of large third-party packs. Here
+// only each pack's name+description sits in the prompt permanently (the
+// Catalog); the full body is loaded lazily when the model calls the single
+// skill_use tool (see Activate).
+//
+// Design shape (each piece is nil-safe / host-agnostic, mirroring the other
+// executus batteries):
+//
+//   - Manifest / ParseManifest — parse+validate a SKILL.md.
+//   - Tree / Pack / LoadPack    — a fetched pack's files, content digest, and
+//     parsed manifest.
+//   - Source (Dir, Git)         — where packs come from; Fetch returns the file
+//     tree and the source's resolved ref.
+//   - Subscription + Store      — the persisted "this host tracks this pack at
+//     this pinned digest" record; Memory is the zero-dep default.
+//   - PackCache                 — content-addressed store of pinned pack trees
+//     so activation never re-fetches; Memory default.
+//   - Syncer                    — checks the tracked ref and records a PENDING
+//     update; applying it is an explicit, separate re-pin (supply-chain guard —
+//     upstream can never silently change what an agent runs).
+//   - Catalog / Activate / Stage — turn a set of resolved packs into a
+//     majordomo agent.Skill (catalog instructions + skill_use tool) and
+//     materialize a pack's files for a sandbox.
+//
+// The host (e.g. mort) supplies policy: which sources are allowed, who may
+// subscribe, and where staged files are mounted. This package supplies only the
+// mechanism.
+package skillpack
+
+import "errors"
+
+// ErrNotFound is returned when a subscription or cached pack lookup misses.
+var ErrNotFound = errors.New("skillpack: not found")
+
+// ErrNoManifest is returned when a fetched tree has no SKILL.md at its root.
+var ErrNoManifest = errors.New("skillpack: tree has no SKILL.md")