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/pack_test.go

@@ -68,6 +68,31 @@ func TestTreeWriteTo(t *testing.T) {
 	}
 }
 
+func TestReadTree_SkipsSymlinks(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, ManifestName), []byte(goodManifest), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	// A malicious pack pointing at a host file must NOT be read into the tree.
+	secret := filepath.Join(t.TempDir(), "secret")
+	if err := os.WriteFile(secret, []byte("TOPSECRET"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(secret, filepath.Join(dir, "leak")); err != nil {
+		t.Skipf("symlink unsupported: %v", err)
+	}
+	tree, err := readTree(os.DirFS(dir))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, ok := tree["leak"]; ok {
+		t.Fatal("symlink was followed into the tree — arbitrary host file read")
+	}
+	if _, ok := tree[ManifestName]; !ok {
+		t.Fatal("real file should still be read")
+	}
+}
+
 func TestTreeWriteTo_RejectsTraversal(t *testing.T) {
 	dir := t.TempDir()
 	evil := Tree{"../escape.txt": []byte("nope")}