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/config.py

@@ -41,6 +41,7 @@ DEFAULTS: Dict[str, Any] = {
     "replay_mode": "mock",        # "mock" (sandboxed prompt) | "fresh" (worktree)
     "evolve_memory": True,        # consolidate CLAUDE.md
     "evolve_skill": True,         # consolidate the managed SKILL.md
+    "llm_mine": True,             # use the backend to mine checkable tasks (real backends)
     # ── adoption / safety ──────────────────────────────────────────────────
     "auto_adopt": False,          # default: stage + require explicit `adopt`
     "managed_skill_name": "skillopt-sleep-learned",