feat: task-aware compression — protect active task context

[ranxianglei]

Summary

• system.ts: Changed Normal level from "Proactively compress" to "After completing a task or sub-task"
• Added task-state awareness: "Do NOT compress content you're actively using for an ongoing task"
• Added pre-compression check: "Will I need this in the next few turns?"

Problem

Model over-compressed from 20% to 6%, losing critical task context. The "Proactively compress" wording encouraged compression even when context was lean and tasks were in progress.

Fix

Two targeted edits to system.ts:

1. Key principle: "compress selectively" + "never compress content you're actively using" + "Compression is for COMPLETED work, not work in progress"
2. Normal level: "After completing a task or sub-task, compress..." instead of "Proactively compress..."