fix: address codex round 2 — revert harvest break + allow lookback 0

- harvest.py: revert break to continue — mtime ordering can diverge
  from embedded ended_at timestamps (copy/touch), so we must check all
  files rather than early-exiting on the first old one
- cycle.py: use `is not None and > 0` so lookback_hours=0 means
  "scan full history" (opt-out of the cutoff)
- __main__.py: propagate --lookback-hours 0 to config as explicit 0

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skillopt_sleep/cycle.py

@@ -148,7 +148,7 @@ def run_sleep_cycle(
         # scan the entire transcript history and trigger massive LLM mining.
         if since is None:
             lookback_hours = cfg.get("lookback_hours", 72)
-            if lookback_hours and lookback_hours > 0:
+            if lookback_hours is not None and lookback_hours > 0:
                 import time
                 ref_time = clock if clock is not None else time.time()
                 cutoff = ref_time - lookback_hours * 3600