feat(backends): rename MergedReadOnlyBackend to MergedSkillsBackend a… (#157)

* feat(backends): rename MergedReadOnlyBackend to MergedSkillsBackend and update documentation for clarity

refactor(paths): simplify ensure_dirs function to create only memory directory eagerly

fix(prompts): update skills availability description for accuracy

test(paths): adjust test to reflect skills directory creation on demand

* refactor(tests): format assertion for skills directory existence in ensure_dirs test

tests/test_paths.py

@@ -102,4 +102,6 @@ class TestEnsureDirsUsesUpdatedPaths:
         paths.ensure_dirs()
 
         assert (new_root / "memory").is_dir()
-        assert (new_root / "skills").is_dir()
+        assert not (
+            new_root / "skills"
+        ).exists()  # skills created on demand by install_skill()