Add concurrent agent limiting to kugetsu CLI

- Add MAX_CONCURRENT_AGENTS (default: 3) to limit concurrent agents
- Implement acquire_agent_slot() and release_agent_slot() with flock
- Wrap cmd_start, cmd_continue, and cmd_delegate with slot management
- cmd_delegate holds slot until background task completes (fire-and-forget + blocking)
- Add basic concurrency tests to test suite

tools/parallel-capacity-test/results/summary_20260331_035345.csv

@@ -0,0 +1,6 @@
+agents,duration,success,failed,timeout,avg_response,stddev,min_response,max_response,peak_cpu,avg_cpu,peak_mem_mb,avg_mem_mb,peak_mem_pct,avg_mem_pct,peak_procs,baseline_mem,mem_per_agent,cost_score
+1,7.01,1,0,0,6.28,0.00,6.28,6.28,0.0,0.0,2546.8,2439.8,0.0,0.0,0,2183.3,363.6,2.55
+2,13.02,2,0,0,9.24,4.46,6.09,12.40,0.0,0.0,2889.0,2659.4,0.0,0.0,0,2189.1,350.0,9.11
+3,8.02,3,0,0,6.33,1.48,4.75,7.69,0.0,0.0,3233.1,2848.9,0.0,0.0,0,2211.8,340.4,8.19
+5,12.04,5,0,0,6.65,2.77,4.71,11.52,0.0,0.0,3911.8,2996.9,0.0,0.0,0,2210.1,340.3,20.49
+8,62.50,0,0,8,60.00,0.00,60.00,60.00,0.0,0.0,4033.0,3940.6,0.0,0.0,0,2245.9,223.4,111.69