fix(curator): default cycle is every 7 days, not 24 hours

Weekly is closer to how skill churn actually works — most agent-created
skills don't change multiple times per day, so a daily review is pure
cost without benefit. Bumping the default to 7 days reduces aux-model
spend while still catching drift and staleness on the timescales that
matter (30d stale, 90d archive).

Changes:
- DEFAULT_INTERVAL_HOURS: 24 -> 168 (7 days)
- config.yaml default: interval_hours: 24 -> 24 * 7
- CLI status line renders as '7d' when interval is a whole-day multiple
- Test `test_old_run_eligible` decoupled from the exact default: it now
  uses 2 * get_interval_hours() so future tweaks don't break it

agent/curator.py

@@ -36,7 +36,7 @@ from tools import skill_usage
 logger = logging.getLogger(__name__)
 
 
-DEFAULT_INTERVAL_HOURS = 24
+DEFAULT_INTERVAL_HOURS = 24 * 7  # 7 days
 DEFAULT_MIN_IDLE_HOURS = 2
 DEFAULT_STALE_AFTER_DAYS = 30
 DEFAULT_ARCHIVE_AFTER_DAYS = 90