Context remaining percentage should account for autocompact buffer

Summary

The "Context Usage" display shows incorrect remaining percentage because it doesn't account for the autocompact buffer.

Current Behavior

When running /context, the display shows:

glm-4.7 · 30k/200k tokens (15%)

This suggests 85% remaining, but the actual details show:

• Free space: 138k (69.2%)
• Autocompact buffer: 33k tokens (16.5%)

Expected Behavior

The remaining percentage should be calculated as:

remaining = (Free space) / Total = 69.2%

Not:

remaining = (Total - Used) / Total = 85%

The autocompact buffer (16.5%) should be subtracted from the displayed remaining percentage since it's reserved for compaction and not available to the user.

Fix

Display: 30k/200k tokens (69%) - accounting for the autocompact buffer.