fix(agent): raise default compaction threshold from 75% to 85%

Compaction was triggering too early because prompt tokens include
system prompt + context files + tools, not just history. Raising
the threshold reduces unnecessary compaction cycles.

internal/config/defaults.go

@@ -9,5 +9,5 @@ const (
 	DefaultMaxMessageChars = 32000
 	DefaultMaxIterations   = 30
 	DefaultTemperature     = 0.7
-	DefaultHistoryShare    = 0.75
+	DefaultHistoryShare    = 0.85
 )