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/manifest_test.go

@@ -61,6 +61,9 @@ func TestParseManifest_Errors(t *testing.T) {
 		"missing desc":       "---\nname: x\n---\nb\n",
 		"bad name uppercase": "---\nname: PdfProcessing\ndescription: d\n---\nb\n",
 		"bad name space":     "---\nname: pdf processing\ndescription: d\n---\nb\n",
+		"bad name leading -": "---\nname: -pdf\ndescription: d\n---\nb\n",
+		"bad name trailing-": "---\nname: pdf-\ndescription: d\n---\nb\n",
+		"bad name double -":  "---\nname: pdf--tools\ndescription: d\n---\nb\n",
 		"bad yaml":           "---\nname: [unclosed\n---\nb\n",
 	}
 	for label, in := range cases {
@@ -80,3 +83,14 @@ func TestParseManifest_LeadingBlanksAndCRLF(t *testing.T) {
 		t.Errorf("got name=%q body=%q", m.Name, m.Body)
 	}
 }
+
+func TestParseManifest_BOM(t *testing.T) {
+	in := append([]byte{0xEF, 0xBB, 0xBF}, []byte("---\nname: bom-ok\ndescription: d\n---\nbody\n")...)
+	m, err := ParseManifest(in)
+	if err != nil {
+		t.Fatalf("BOM-prefixed SKILL.md should parse: %v", err)
+	}
+	if m.Name != "bom-ok" {
+		t.Errorf("name = %q", m.Name)
+	}
+}