fix(agent): prevent team V2 leads from manually creating tasks before spawn

Strengthen TEAM.md prompt with WRONG/CORRECT examples and NEVER wording
to stop models from calling `team_tasks create` before `spawn` in V2.

Prompt changes:
- V2 leads: explicit WRONG/CORRECT pattern, NEVER create before spawn
- V1 leads: separate workflow with manual create→spawn instructions
- team_tasks summary: de-emphasize "create", highlight auto-creation
- spawn team_task_id: clarify "omit to auto-create (recommended)"

Backend guards:
- Reject spawn with in-progress team_task_id (prevents reuse)
- Log warning on early claim race instead of silently ignoring

internal/tools/subagent_spawn_tool.go

@@ -90,7 +90,7 @@ func (t *SpawnTool) Parameters() map[string]any {
 		}
 		props["team_task_id"] = map[string]any{
 			"type":        "string",
-			"description": "Team task ID to auto-complete when task finishes (for team workflows)",
+			"description": "Omit to auto-create a task (recommended). Only set for dependency chains with blocked_by tasks.",
 		}
 		props["estimated_duration"] = map[string]any{
 			"type":        "number",