fix(registry): fall back to default SIN_SKILLS_DIR when env var is unset

cmd/sin-code/internal/mcpclient/registry.go

@@ -14,7 +14,7 @@ import (
 // tool-name prefixes ("websearch__search", "browser__navigate", ...), which
 // the permission matrix gates via the "mcp" policy class.
 func DefaultServers() []ServerConfig {
-	skillsDir := os.Getenv("SIN_SKILLS_DIR")
+	skillsDir := skillsDirOrDefault()
 	py := func(repo string) ServerConfig {
 		name := shortName(repo)
 		cfg := ServerConfig{Name: name, Transport: "stdio"}
@@ -85,3 +85,14 @@ func shortName(repo string) string {
 	}
 	return repo
 }
+
+// skillsDirOrDefault returns the configured SIN_SKILLS_DIR or the default
+// local share location used by skillmgr. This keeps the registry in sync
+// with where skillmgr actually installs skills.
+func skillsDirOrDefault() string {
+	if d := os.Getenv("SIN_SKILLS_DIR"); d != "" {
+		return d
+	}
+	home, _ := os.UserHomeDir()
+	return filepath.Join(home, ".local", "share", "sin-code", "skills")
+}

cmd/sin-code/internal/mcpclient/registry_test.go

@@ -35,6 +35,9 @@ func TestDefaultServersWebsearchUsesLocalBinaryWhenPresent(t *testing.T) {
 }
 
 func TestDefaultServersWebsearchFallsBackToPathBinary(t *testing.T) {
+	// Use a HOME that has no sin-code skills checkout so the default skills
+	// dir check fails and the registry falls back to the binary on PATH.
+	t.Setenv("HOME", t.TempDir())
 	t.Setenv("SIN_SKILLS_DIR", "")
 	for _, s := range DefaultServers() {
 		if s.Name != "websearch" {
@@ -47,3 +50,27 @@ func TestDefaultServersWebsearchFallsBackToPathBinary(t *testing.T) {
 	}
 	t.Fatal("websearch server not found in DefaultServers")
 }
+
+func TestDefaultServersWebsearchUsesDefaultSkillsDir(t *testing.T) {
+	home := t.TempDir()
+	bin := filepath.Join(home, ".local", "share", "sin-code", "skills", "web_search_bundle", "sin-websearch")
+	if err := os.MkdirAll(filepath.Dir(bin), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(bin, []byte("#!/bin/sh\necho fake"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	t.Setenv("SIN_SKILLS_DIR", "")
+	t.Setenv("HOME", home)
+	for _, s := range DefaultServers() {
+		if s.Name != "websearch" {
+			continue
+		}
+		if s.Command != bin {
+			t.Fatalf("websearch command should use default skills dir binary %q, got %q", bin, s.Command)
+		}
+		return
+	}
+	t.Fatal("websearch server not found in DefaultServers")
+}