feat: recursive skill discovery + tree-url type-root folding

- skills are found at any depth under the skills dir, so category-nested
  layouts (skills/<category>/<id>/SKILL.md) vendor correctly; the folder
  basename is the upstream id and the outermost SKILL.md wins over anything
  nested inside a skill folder
- web-tree urls pointing directly at a type directory (.../tree/main/skills)
  now map that directory as the type root instead of nesting it under the
  default layout
- verified against the real github.com/mattpocock/skills repository
  (36 skills across categories; include-selection by basename)

internal/discovery/nested_test.go

@@ -0,0 +1,73 @@
+package discovery
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func writeTree(t *testing.T, root string, files map[string]string) {
+	t.Helper()
+	for rel, content := range files {
+		p := filepath.Join(root, filepath.FromSlash(rel))
+		os.MkdirAll(filepath.Dir(p), 0o755)
+		os.WriteFile(p, []byte(content), 0o644)
+	}
+}
+
+func TestScanNestedSkillCategories(t *testing.T) {
+	dir := t.TempDir()
+	writeTree(t, dir, map[string]string{
+		"skills/flat-skill/SKILL.md":            "---\nname: Flat\n---\n",
+		"skills/engineering/tdd/SKILL.md":       "---\nname: TDD\n---\n",
+		"skills/productivity/grill-me/SKILL.md": "---\nname: Grill Me\n---\n",
+		"skills/engineering/tdd/reference.md":   "extra artifact",
+		"skills/deprecated/old/README.md":       "no skill here",
+		"commands/deploy.md":                    "---\nname: Deploy\n---\n",
+	})
+
+	items, err := Scan(&FsTree{Root: dir}, Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := map[string]Item{}
+	for _, it := range items {
+		got[it.UpstreamID] = it
+	}
+	for _, want := range []string{"flat-skill", "tdd", "grill-me"} {
+		it, ok := got[want]
+		if !ok {
+			t.Fatalf("skill %q not discovered (items: %+v)", want, items)
+		}
+		if it.Type != "skills" {
+			t.Errorf("%q type = %q", want, it.Type)
+		}
+	}
+	if _, ok := got["old"]; ok {
+		t.Error("folder without SKILL.md must not be a skill")
+	}
+	tdd := got["tdd"]
+	if tdd.RelPath != "skills/engineering/tdd" {
+		t.Errorf("tdd RelPath = %q", tdd.RelPath)
+	}
+	if got["deploy"].Type != "commands" {
+		t.Error("flat command types must still be discovered")
+	}
+}
+
+func TestScanOuterSkillWinsOverNested(t *testing.T) {
+	dir := t.TempDir()
+	writeTree(t, dir, map[string]string{
+		"skills/bundle/SKILL.md":            "---\nname: Bundle\n---\n",
+		"skills/bundle/inner/SKILL.md":      "---\nname: Inner\n---\n",
+		"skills/bundle/inner/deep/SKILL.md": "---\nname: Deep\n---\n",
+	})
+
+	items, err := Scan(&FsTree{Root: dir}, Config{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(items) != 1 || items[0].UpstreamID != "bundle" {
+		t.Fatalf("outermost skill must win, got %+v", items)
+	}
+}