feat(#25): Subordinate delegation loop prevention and depth limits

[Nafania]

Summary

• Add max_agent_depth to AgentConfig (default: 5) — hard limit on delegation depth
• Refuse delegation when depth limit reached with clear error message
• Detect same-profile delegation loops: if same profile delegates to itself 3+ times consecutively, delegation is refused
• Add depth awareness to agent info prompt (shows current depth, max, remaining levels)
• Show subordinate status in agent info prompt

Test plan

• test_refuses_at_max_depth — verifies depth limit enforcement
• test_allows_under_max_depth — verifies normal delegation works
• test_refuses_after_3_same_profile_delegations — verifies loop detection
• test_allows_different_profile — verifies different profiles bypass loop check