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>

CLAUDE.md

@@ -85,6 +85,14 @@ BATTERIES (opt-in siblings, each nil-safe + a default):
              (throttled Save/Complete/Fail) + Memory
   budget/    DBBudget rolling-7d + NoOp (run.Budget);    [P4 ✓]
              BudgetStorage iface + Memory default
+  skillpack/ SKILL.md-subscription battery: Manifest +   [P5 ✓]
+             Source (Dir/Git) + Subscription/Store +
+             content-addressed PackCache + Syncer
+             (pending-only; Apply re-pins) + Activate →
+             majordomo agent.Skill (catalog + skill_use,
+             progressive disclosure) + Memory defaults.
+             NOT executus/skill (saved-agent noun) nor
+             majordomo/skill (eager capability bundle).
 
 contrib/store/   SECOND module (+ modernc.org/sqlite):   [P4 ✓]
   pure-Go SQLite impls of ALL store seams: budget +