fix(agent): address critical issues from PR #1485 review

1. Prevent double-close panic on Stop() by using sync.Once in Scheduler,
   Brain, and Sentinel; remove duplicate Stop() call in main.go
2. Add trade quantity (100k) and leverage (125x) sanity caps to prevent
   LLM hallucinations or input errors from reaching the exchange
3. Mask secrets in onboarding setup state — only store "****" markers in
   SystemConfig instead of plaintext API keys/secrets

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agent/brain.go

@@ -17,6 +17,7 @@ type Brain struct {
 	logger        *slog.Logger
 	http          *http.Client
 	stopCh        chan struct{}
+	stopOnce      sync.Once
 	recentSignals sync.Map // debounce
 }
 
@@ -29,7 +30,7 @@ func NewBrain(agent *Agent, logger *slog.Logger) *Brain {
 	}
 }
 
-func (b *Brain) Stop() { close(b.stopCh) }
+func (b *Brain) Stop() { b.stopOnce.Do(func() { close(b.stopCh) }) }
 
 // cleanStaleSignals removes debounce entries older than 30 minutes.
 func (b *Brain) cleanStaleSignals() {