Add `contextStrategy: 'compact'` as a zero-LLM-cost alternative to `summarize`.
Instead of making an LLM call to compress everything into prose, it selectively
compresses old turns using structural rules:
- Preserve tool_use blocks (agent decisions) and error tool_results
- Replace long tool_result content with compact markers including tool name
- Truncate long assistant text blocks with head excerpts
- Keep recent turns (configurable via preserveRecentTurns) fully intact
- Detect already-compressed markers from compressToolResults to avoid double-processing
Closes#111
JackChen
MrAvalonApple