fix(skills): install universal agent to ~/.agents/skills

The Universal agent host installed user-scoped skills to
~/.config/agents/skills, which is not the location scanned by the
majority of compliant clients (Copilot CLI, Pi, OpenCode). Per the
agentskills.io cross-client convention, user-level skills for
compliant clients live under ~/.agents/skills.

Switch UserDir for the universal agent to the shared project skills
directory constant, matching the convention used by Warp and Cline.
Add regression tests for both project and user scope.

Fixes cli#13494

Author: toller892

internal/skills/registry/registry.go

@@ -299,7 +299,7 @@ var Agents = []AgentHost{
 		ID:         "universal",
 		Name:       "Universal",
 		ProjectDir: sharedProjectSkillsDir,
-		UserDir:    ".config/agents/skills",
+		UserDir:    sharedProjectSkillsDir,
 	},
 	{
 		ID:         "warp",

internal/skills/registry/registry_test.go

@@ -125,6 +125,25 @@ func TestInstallDir(t *testing.T) {
 			homeDir: "/home/monalisa",
 			wantDir: filepath.Join("/tmp/monalisa-repo", ".agents", "skills"),
 		},
+		{
+			// Issue #13494: Universal must use the shared .agents/skills dir
+			// at user scope so compliant clients (Copilot, Pi, OpenCode) pick up
+			// skills per the agentskills.io cross-client convention.
+			name:    "universal project scope",
+			hostID:  "universal",
+			scope:   ScopeProject,
+			gitRoot: "/tmp/monalisa-repo",
+			homeDir: "/home/monalisa",
+			wantDir: filepath.Join("/tmp/monalisa-repo", ".agents", "skills"),
+		},
+		{
+			name:    "universal user scope",
+			hostID:  "universal",
+			scope:   ScopeUser,
+			gitRoot: "/tmp/monalisa-repo",
+			homeDir: "/home/monalisa",
+			wantDir: filepath.Join("/home/monalisa", ".agents", "skills"),
+		},
 		{
 			name:    "project scope without git root",
 			hostID:  "github-copilot",