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>

skillopt_sleep/backend.py

@@ -1415,6 +1415,13 @@ def get_backend(
         return AzureResponsesBackend(deployment=model, endpoints=eps)
     if n in {"copilot", "github_copilot", "copilot_cli", "gh_copilot"}:
         return CopilotCliBackend(model=model)
+    if n in {"handoff", "session", "file"}:
+        # Lazy import: handoff_backend imports CliBackend from this module.
+        from skillopt_sleep.handoff_backend import HandoffBackend
+        hdir = os.environ.get("SKILLOPT_SLEEP_HANDOFF_DIR", "") or os.path.join(
+            project_dir or os.getcwd(), ".skillopt-sleep-handoff"
+        )
+        return HandoffBackend(model=model, handoff_dir=hdir)
     return MockBackend()