feat: vendor add end-to-end with deterministic lockfile v2

- go-git in-process clone, default-branch/tag resolution, optional token auth
- web-tree URL normalization (github/gitlab) with implied discovery root
- discovery of skills/commands/agents/mcp, lenient frontmatter with warnings
- selection: all by default, --include with missing-entry hard errors
- external/<source>/<type>/ materialization preserving exec bits
- lockfile v2: deterministic JSON, pinned url/ref/rev, inventory, warnings
- offline black-box e2e suite against local fixture git repositories

internal/discovery/frontmatter_test.go

@@ -0,0 +1,54 @@
+package discovery
+
+import "testing"
+
+func TestParseFrontmatter(t *testing.T) {
+	cases := []struct {
+		name    string
+		in      string
+		want    Frontmatter
+		warnLen int
+	}{
+		{
+			name: "complete block",
+			in:   "---\nname: Test Skill\ndescription: does things\ntags: a, b\nrequires: c,d\n---\nbody\n",
+			want: Frontmatter{Name: "Test Skill", Description: "does things", Tags: []string{"a", "b"}, Requires: []string{"c", "d"}},
+		},
+		{
+			name: "no frontmatter",
+			in:   "# Just a file\nbody\n",
+			want: Frontmatter{},
+		},
+		{
+			name:    "unclosed block",
+			in:      "---\nname: Broken\ndescription: never closed\n",
+			want:    Frontmatter{Name: "Broken", Description: "never closed"},
+			warnLen: 1,
+		},
+		{
+			name:    "line missing colon",
+			in:      "---\nname: X\nbroken line without colon\n---\n",
+			want:    Frontmatter{Name: "X"},
+			warnLen: 1,
+		},
+		{
+			name: "crlf line endings",
+			in:   "---\r\nname: Win\r\ndescription: windows author\r\n---\r\n",
+			want: Frontmatter{Name: "Win", Description: "windows author"},
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			fm, warnings := ParseFrontmatter([]byte(tc.in))
+			if fm.Name != tc.want.Name || fm.Description != tc.want.Description {
+				t.Errorf("got %+v, want %+v", fm, tc.want)
+			}
+			if len(fm.Tags) != len(tc.want.Tags) || len(fm.Requires) != len(tc.want.Requires) {
+				t.Errorf("tags/requires mismatch: got %+v want %+v", fm, tc.want)
+			}
+			if len(warnings) != tc.warnLen {
+				t.Errorf("warnings = %v (len %d), want len %d", warnings, len(warnings), tc.warnLen)
+			}
+		})
+	}
+}