merge: custom skill sources

# Conflicts:
#	skiff/README.md
#	skiff/cli.py
#	skiff/project.py

tests/test_select.py

@@ -139,7 +139,7 @@ class SelectCommandTests(unittest.TestCase):
                 patch.object(
                     cli,
                     "_install_skill",
-                    side_effect=lambda name, targets, project_root: installed.append(name),
+                    side_effect=lambda name, targets, project_root, **kwargs: installed.append(name),
                 ),
             ):
                 cli.cmd_select(args)
@@ -195,7 +195,7 @@ class SelectCommandTests(unittest.TestCase):
             links["claude"].mkdir(parents=True)
 
             with (
-                patch.object(cli, "_ensure_external_fetched"),
+                patch.object(cli, "_ensure_source_fetched"),
                 patch.object(cli, "resolve_skill_source", return_value=(skill, "owned")),
                 patch.object(
                     cli,