Adds two new config options to memoryFlush:
- hardThresholdTokens: token count that triggers auto-execution
- hardThresholdCommand: shell command to run (e.g., kernle checkpoint save)
Flow:
1. Soft threshold (existing): prompts agent to save with context
2. Hard threshold (new): auto-executes command without agent involvement
This provides a safety net - agents get a chance to save with context at
soft threshold, but if they miss it, the hard threshold ensures state is
preserved before compaction.
Example config:
```json
{
"memoryFlush": {
"softThresholdTokens": 100000,
"prompt": "Save your state to Kernle...",
"hardThresholdTokens": 120000,
"hardThresholdCommand": "kernle -a agent checkpoint save 'auto-backup'"
}
}
```
