fix(skillpack): address review — symlink read, git arg-injection, dup-subscribe, nil panics

Real issues from the PR review:
- security: readTree now skips symlinks (a pack with SKILL.md -> /etc/passwd
  or scripts/x -> ../../.ssh/id_rsa could read host files); covers file and
  dir symlinks, incl. within a git subpath
- security: GitSource rejects url/ref beginning with '-' (git arg injection)
  and clones with '--' separator; --filter=blob:none (blobless partial clone)
  instead of full-history clone
- correctness: Subscribe no longer swallows a non-ErrNotFound store error from
  GetByName (would create a duplicate subscription); handles *GitSource as well
  as GitSource in the URL/subpath extraction
- correctness: pinTo no longer renames a subscription, so Apply can't silently
  collide two subscriptions when an upstream pack changes its name
- validation: isKebab rejects leading/trailing/consecutive hyphens; BOM-
  prefixed SKILL.md now parses (matches the doc comment)
- robustness: Catalog/Activate/renderPackBody/Stage guard nil/malformed packs
- test cleanup: Syncer.Store field renamed Cache (collided with the Store
  interface); test NewID returns distinct ids
- tests: symlink-skip, BOM, strict-kebab, nil-pack-safety

Deferred (advisory perf, documented): PackCache stores raw trees so activation
re-parses; CheckAll is serial. Both fine at expected scale.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

skillpack/subscription.go

@@ -50,9 +50,10 @@ func (s *Subscription) HasPending() bool {
 }
 
 // pinTo advances the active pin to a fetched pack and clears any pending state.
-// Used by initial pin and by Apply.
+// Used by initial pin and by Apply. It does NOT set Name: a subscription's name
+// is its stable host handle, fixed at Subscribe time — letting an upstream pack
+// rename move it would silently collide with another subscription on 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