feat(sleep): 3-way train/val/test split + gate_mode on|off

Data-split refactor (the anti-overfitting foundation the user asked for):
  - TaskRecord gains split∈{train,val,test} and origin∈{real,dream}.
  - assign_splits: real tasks deterministically split into val/test (disjoint);
    DREAM-augmented tasks (origin='dream') NEVER enter val/test — they only go to
    train. val gates updates; test is the final held-out measure.
  - gbrain loader maps its held-out.jsonl -> test, benchmark.jsonl -> train/val,
    so the gbrain held-out stays the true final score.
  - consolidate(): train drives reflect, val gates; adds gate_mode='off' (greedy,
    no hard filter) reporting val movement (greedy_improved/regressed/flat).
  - run_gbrain/transfer/experiment score on test (val fallback); run_gbrain gains
    --gate on|off. Legacy replay/holdout names normalized.

New test proves dream tasks never land in val/test. 21 tests pass; mock
experiment + gate=off both green.

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

skillopt/sleep/types.py

@@ -61,7 +61,16 @@ class TaskRecord:
     judge: Dict[str, Any] = field(default_factory=dict)  # gbrain-style rule judge
     tags: List[str] = field(default_factory=list)
     source_sessions: List[str] = field(default_factory=list)
-    split: str = "replay"             # replay (train) | holdout (test)
+    # split ∈ {train, val, test}.  val + test come ONLY from real mined tasks and
+    # never overlap (val gates updates, test is the final held-out measure). train
+    # may be dream-augmented (see origin).  Legacy values replay->train,
+    # holdout->val are normalized on load.
+    split: str = "train"
+    # origin ∈ {real, dream}.  'real' = mined from the user's actual sessions;
+    # 'dream' = synthetic/augmented for the training pool. Dream tasks are NEVER
+    # allowed into val/test, which is the anti-overfitting guarantee.
+    origin: str = "real"
+    derived_from: str = ""            # for dream tasks: the real task id it varies
 
     def to_dict(self) -> Dict[str, Any]:
         return asdict(self)