Feature request: Configurable autocompact buffer size

Summary

Make the autocompact buffer size configurable instead of fixed at ~45k tokens (22.5% of context).

Use Case

Power users running long sessions who understand the tradeoff between buffer size and context preservation. A smaller buffer (e.g., 10k tokens) would provide more working space at the cost of more aggressive compaction.

Suggested Implementation

• Add a setting in /config like "Autocompact buffer tokens" or a percentage slider
• Or expose via settings.json as autocompactBufferTokens or autocompactBufferPercent

Current Behavior

• Fixed 45k token buffer (~22.5% of 200k context)
• Cannot be adjusted by users

Desired Behavior

• User-configurable buffer size (absolute tokens or percentage)
• Default can remain at 45k for safety, but allow reduction for those who want it