fix(agent): implement Critical flag, complete tools.SubTurnConfig, remove redundant subTurnResults

- Critical flag was declared but never acted on; non-critical SubTurns
  now break out of the iteration loop when IsParentEnded() returns true
- tools.SubTurnConfig was missing Critical/Timeout/MaxContextRunes,
  making those fields unreachable from the tools layer; added fields and
  wired them through AgentLoopSpawner.SpawnSubTurn
- Removed subTurnResults sync.Map from AgentLoop — it was a redundant
  alias for the same channel already stored in turnState.pendingResults;
  dequeuePendingSubTurnResults now reads directly via activeTurnStates
- Replace hardcoded concurrencySem size 5 with maxConcurrentSubTurns constant
- Update affected tests to match new dequeuePendingSubTurnResults API

pkg/tools/subagent.go

@@ -22,7 +22,10 @@ type SubTurnConfig struct {
 	SystemPrompt string
 	MaxTokens    int
 	Temperature  float64
-	Async        bool // true for async (spawn), false for sync (subagent)
+	Async        bool          // true for async (spawn), false for sync (subagent)
+	Critical     bool          // continue running after parent finishes gracefully
+	Timeout      time.Duration // 0 = use default (5 minutes)
+	MaxContextRunes int        // 0 = auto, -1 = no limit, >0 = explicit limit
 }
 
 type SubagentTask struct {