feat: make autoCompactThreshold configurable via global config

[luojiyin1987]

Summary

• Add autoCompactThreshold to GlobalConfig type and GLOBAL_CONFIG_KEYS
• Replace hardcoded threshold (0.92) with configurable value via getAutoCompactThresholdRatio()
• Users can now customize the auto-compact trigger ratio in ~/.kode.json

Fixes #143

Configuration Example

{
  "autoCompactThreshold": 0.80
}

Value	Effect	Use Case
0.80	Earlier, more conservative	Small context models (e.g. deepseek 131k)
0.85	Balanced	Medium context models
0.92	Default	Large context models (e.g. Claude 200k)

🤖 Generated with Claude Code