Increased Startup Performance Telemetry Sampling

What: Startup performance telemetry sampling rate increased from 0.1% to 0.5% of CLI invocations.

Details:

• Constant changed from OP9 = 0.001 to Xa2 = 0.005
• Provides 5x more data points for analyzing real-world startup performance
• Can still be forced on with CLAUDE_CODE_PROFILE_STARTUP=1 environment variable
• Evidence: Xa2 = 0.005 at line 2661, sampling logic at line 2674