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/vendor/add.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -51,6 +52,7 @@ func Add(workDir string, opts AddOptions, stdout io.Writer) error {
 	if err != nil {
 		return err
 	}
+	cfg := discoveryFromTreeRoot(root)
 
 	repo, err := gitsource.Clone(normalized, opts.Ref)
 	if err != nil {
@@ -61,7 +63,6 @@ func Add(workDir string, opts AddOptions, stdout io.Writer) error {
 		return err
 	}
 
-	cfg := lockfile.Discovery{Root: root}
 	items, err := discovery.Scan(tree, discovery.FromLockfileDiscovery(cfg))
 	if err != nil {
 		return err
@@ -117,6 +118,35 @@ func Add(workDir string, opts AddOptions, stdout io.Writer) error {
 	return nil
 }
 
+// discoveryFromTreeRoot maps a pasted tree sub-path onto discovery config.
+// A sub-path ending in a standard type directory (e.g. ".../tree/main/skills")
+// selects that directory as the type root; anything else becomes the
+// discovery root under which the standard layout is expected.
+func discoveryFromTreeRoot(root string) lockfile.Discovery {
+	if root == "" {
+		return lockfile.Discovery{}
+	}
+	parent, base := path.Split(strings.TrimSuffix(root, "/"))
+	parent = strings.TrimSuffix(parent, "/")
+	d := lockfile.Discovery{Root: parent}
+	switch base {
+	case lockfile.TypeSkill:
+		d.SkillsDir = base
+	case lockfile.TypeCommand:
+		d.CommandsDir = base
+	case lockfile.TypeAgent:
+		d.AgentsDir = base
+	case lockfile.TypeMcp:
+		d.McpDir = base
+	default:
+		return lockfile.Discovery{Root: root}
+	}
+	if d == (lockfile.Discovery{}) {
+		return lockfile.Discovery{}
+	}
+	return d
+}
+
 func checkIncludeEntries(items []discovery.Item, include []string, name string) error {
 	available := map[string]bool{}
 	for _, it := range items {