fix(models): restore legacy Metadata = TrackMetadata alias for backward compat

tests/test_track_state_schema.py imports 'from src.models import
Metadata' and uses it as a dataclass (e.g. 'Metadata(id=..., created_at=...)').
After Phase 5, models.Metadata was undefined and __getattr__ returned
the type alias from src.type_aliases (which is dict[str, Any]). The
test then failed with 'TypeError: dict.__init__() got an unexpected
keyword argument created_at'.

This commit restores the legacy 'Metadata = TrackMetadata' alias at
the top of models.py so 'from src.models import Metadata' resolves to
the TrackMetadata dataclass (the original behavior). New code should
import directly: 'from src.mma import TrackMetadata'.

Also removes the now-redundant __getattr__ entry for Metadata (it's
eager now).

Tests verified:
  tests/test_track_state_schema.py (5/5 PASS; was 2/5 before this fix)

src/models.py

@@ -35,6 +35,17 @@ import sys
 
 from typing import Any, Dict, List
 
+from src.mma import TrackMetadata
+
+# Legacy alias: the original models.py had a TrackMetadata dataclass named
+# 'Metadata' (before Phase 3a moved it to src.mma.py). Existing tests +
+# consumers use 'from src.models import Metadata' expecting the dataclass.
+# We re-export the dataclass here so the legacy import path resolves to
+# the same object. The Metadata TYPE ALIAS (from src.type_aliases) is the
+# boundary wire type; legacy consumers wanting the dataclass should
+# migrate to 'from src.mma import TrackMetadata'.
+Metadata = TrackMetadata  # noqa: F401 — legacy class name re-export
+
 
 DEFAULT_TOOL_CATEGORIES: Dict[str, List[str]] = {
  "General": ["read_file", "list_directory", "search_files", "get_tree", "get_file_summary"],