fix(agent): prevent "..." fallback when iteration budget exhausted

Add early detection and graceful degradation when the model exhausts its
iteration budget without producing a text response:

- Inject 75% budget nudge telling model to start summarizing
- Strip all tool schemas on final iteration to force text-only response
- Adaptive web_fetch maxChars (60K→20K→10K) via iteration progress context
- Bump DefaultMaxIterations 20→30 for web-research workloads

internal/config/defaults.go

@@ -7,7 +7,7 @@ const (
 	DefaultContextWindow   = 200000
 	DefaultMaxTokens       = 8192
 	DefaultMaxMessageChars = 32000
-	DefaultMaxIterations   = 20
+	DefaultMaxIterations   = 30
 	DefaultTemperature     = 0.7
 	DefaultHistoryShare    = 0.75
 )