feat(sleep): add handoff backend — session-executed model calls, no API subprocess (#125)

Adds --backend handoff: the engine runs all deterministic stages and
outsources attempt/judge/reflect to prompt/answer files an interactive
agent session fills between runs (exit 3 = pending batch, re-run to
resume). Deterministic replay + the prompt-hash answer cache make resume
stateless; sentinel detection aborts any call built from unanswered
output so placeholders never reach scores or staging. Session digests
and mined tasks are pinned per night (secret-redacted) so the sessions
answering prompts cannot shift the task set, and LLM mining is routed
through the same handoff files. Ships a /skillopt-sleep-handoff Claude
Code command that answers each prompt in a fresh-context subagent to
protect the held-out gate.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -24,6 +24,9 @@ logs/
 external/
 # SkillOpt-Sleep runtime state (staging proposals, config, diagnostics, cron logs)
 .skillopt-sleep/
+# SkillOpt-Sleep handoff-backend round data (prompts/answers derived from transcripts)
+.skillopt-sleep-handoff/
+.skillopt-sleep-handoff.night*.done/
 
 /BabyVision/
 /MMRB/