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/activation_test.go

@@ -86,6 +86,25 @@ func TestActivate_Empty(t *testing.T) {
 	}
 }
 
+func TestNilPackElementsAreSafe(t *testing.T) {
+	packs := []*Pack{nil, mustPack(t, "real", "b", nil), {Manifest: nil}}
+	// Neither Catalog nor Activate may panic on nil / malformed elements.
+	if got := Catalog(packs); !strings.Contains(got, "real") {
+		t.Errorf("catalog should include the valid pack and skip nils: %q", got)
+	}
+	sk := Activate(packs, "")
+	if sk == nil {
+		t.Fatal("a valid pack among nils should still activate")
+	}
+	if _, ok := sk.Tools().Get("skill_use"); !ok {
+		t.Error("skill_use missing")
+	}
+	// All-nil activates to nothing rather than panicking.
+	if Activate([]*Pack{nil, {Manifest: nil}}, "") != nil {
+		t.Error("only-nil packs should activate to nil")
+	}
+}
+
 func TestResolveFromCache(t *testing.T) {
 	ctx := context.Background()
 	cache := NewMemoryPackCache()