Add context usage warning before auto-compact

Feature Request: Context Usage Warning Before Auto-Compact

Problem: Users aren't warned before auto-compact happens, losing conversation context unexpectedly during long coding sessions.

Proposed Solution: Add a configurable warning when context usage reaches a specified threshold (e.g., 90%).

Use Case:

• Long debugging sessions where conversation history is critical
• Complex multi-step implementations where losing context breaks flow
• Users who want to proactively use /memory before auto-compact

Suggested Implementation:

• Setting: CLAUDE_CODE_CONTEXT_WARNING_THRESHOLD=0.9 (default 90%)
• Display: "⚠️ Context 90% full - consider using /memory to save important info"
• Optional: Show current context usage percentage in status

Benefits:

• Prevents unexpected context loss
• Encourages proactive memory usage
• Improves user experience for power users

Alternative: Could also be implemented as a status indicator showing current context usage percentage.