feat(sleep): optimizer/target model split, transfer experiment, LLM miner

Three additions driven by the goal of price-aware, model-flexible sleep:

1. DualBackend + build_backend(): route attempt->TARGET model and
   reflect/judge->OPTIMIZER model (SkillOpt's target-vs-optimizer split).
   gbrain runner gains --optimizer-backend/-model + --target-backend/-model.

2. run_transfer.py: sleep-scenario cross-model transfer. Optimize a skill on a
   SOURCE model (e.g. cheap haiku), freeze it, evaluate held-out on a TARGET
   model (e.g. expensive sonnet) with no further optimization — plus a direct
   reference. Mirrors the SkillOpt paper's transfer table; quantifies the
   "optimize cheap overnight, deploy anywhere" value prop.

3. llm_miner.py: turn real harvested transcripts into TaskRecords WITH checkable
   rule/rubric judges, wired into the cycle for non-mock backends, so real-data
   lift becomes measurable (heuristic miner remains the no-API fallback).
   Fixed a str.format brace bug the new unit test caught.

19 tests pass.

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

skillopt/sleep/cycle.py

@@ -125,11 +125,22 @@ def run_sleep_cycle(
             limit=cfg.get("max_tasks_per_night", 40) * 3,
         )
         n_sessions = len(digests)
+        # When a real backend is configured, use it to mine checkable tasks from
+        # the transcripts (rubric/rule judges); otherwise fall back to the
+        # heuristic miner (no API, no checkable reference).
+        llm_miner = None
+        if cfg.get("backend", "mock") != "mock" and cfg.get("llm_mine", True):
+            try:
+                from skillopt.sleep.llm_miner import make_llm_miner
+                llm_miner = make_llm_miner(backend, max_tasks=cfg.get("max_tasks_per_night", 40))
+            except Exception:
+                llm_miner = None
         tasks = mine(
             digests,
             max_tasks=cfg.get("max_tasks_per_night", 40),
             holdout_fraction=cfg.get("holdout_fraction", 0.34),
             seed=cfg.get("seed", 42),
+            llm_miner=llm_miner,
         )
 
     # ── live skill/memory docs ───────────────────────────────────────────