feat: dual-trigger nudge wording — proactive after use + pressure rises

[ranxianglei]

Summary

• system.ts Normal level: "Be frugal" → "Proactively compress finished outputs (agent results, verbose commands, large tool outputs) into summaries after you've finished using them"
• Two triggers coexist: proactive (Normal) + pressure-based (Elevated/Critical)
• Config: nudgeFrequency 8 → 6

Changes

• lib/prompts/system.ts line 19: Updated Normal pressure level wording