Agent tool spawns orphan agent when team_name references nonexistent team

Bug

When the Agent tool is called with team_name pointing at a nonexistent team, the tool returns an error message but still spawns the agent. The orphan agent runs to completion and its output never routes back to the caller.

Steps to Reproduce

1. Call Agent(team_name: "my-team", prompt: "do something") where "my-team" does not exist (e.g., because TeamCreate silently renamed it — see related bug)
2. Observe: error message returned ("Team does not exist")
3. Observe: agent_spawned event in activity log — agent runs to completion as orphan
4. Agent output is lost (never delivered to caller)

Expected Behavior

If the target team does not exist, the Agent call should fail cleanly without spawning any agent.

Actual Behavior

Error message returned AND agent spawned. The orphan runs to completion (observed: 94-162 seconds), consuming tokens with no output delivery.

Impact

• Duplicate work and wasted tokens (observed: 4 agents spawned for work of 2)
• Silent failures — caller believes the call failed but orphan is running
• Compounds with TeamCreate silent rename bug: requested name is ignored → subsequent Agent calls with that name fail → orphan spawns

Environment

• Claude Code CLI
• Observed: 2026-04-08