feat: -x/--exclude to skip folders during --repair/--retag-from-path

Repeatable -x/--exclude NAME skips any artist- or source-level folder whose name
matches (case-insensitive) when walking the library, so hand-curated folders like
/media/music/Unsorted or .../playlists are left untouched. Threaded through
_iter_source_files -> repair_library / retag_library_from_path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_repair.py

@@ -159,6 +159,23 @@ def test_repair_library_missing_root():
     assert mf.repair_library("/no/such/dir", dry_run=False) == (0, 0)
 
 
+def test_repair_library_exclude_skips_folders(tmp_path, monkeypatch):
+    root = tmp_path
+    (root / "Daft Punk" / "youtube").mkdir(parents=True)
+    (root / "Daft Punk" / "youtube" / f"A [{YT_ID}].opus").write_text("x")
+    (root / "Unsorted" / "youtube").mkdir(parents=True)          # excluded artist folder
+    (root / "Unsorted" / "youtube" / f"B [{YT_ID}].opus").write_text("x")
+    (root / "Ephixa" / "playlists").mkdir(parents=True)          # excluded source folder
+    (root / "Ephixa" / "playlists" / f"C [{YT_ID}].opus").write_text("x")
+
+    visited = []
+    monkeypatch.setattr(mf, "repair_file",
+                        lambda path, source, dry_run: visited.append(path) or ["x"])
+    scanned, _ = mf.repair_library(str(root), dry_run=False, exclude=["unsorted", "playlists"])
+    assert scanned == 1
+    assert visited and "Daft Punk" in visited[0]
+
+
 # ---- offline retag-from-path ----
 def test_title_from_filename():
     assert mf._title_from_filename(f"Song [{YT_ID}].opus") == "Song"