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_035130.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
+1,1.01,0,1,0,0.01,0.00,0.01,0.01,0.0,0.0,2177.1,2177.1,0.0,0.0,0,2177.1,-0.0,0.00
+2,1.02,0,2,0,0.00,0.00,0.00,0.00,0.0,0.0,2175.7,2175.5,0.0,0.0,0,2175.1,0.3,0.00
+3,1.02,0,3,0,0.00,0.00,0.00,0.00,0.0,0.0,2175.2,2175.2,0.0,0.0,0,2175.0,0.1,0.00
+5,1.02,0,5,0,0.00,0.00,0.00,0.00,0.0,0.0,2174.8,2174.8,0.0,0.0,0,2174.7,0.0,0.00
+8,1.02,0,8,0,0.00,0.00,0.00,0.00,0.0,0.0,2176.0,2175.3,0.0,0.0,0,2174.7,0.2,0.00