[FEATURE] Add auto-compact threshold for long sessions

Problem

Currently opencode TUI has no automatic context compression. After long sessions, context grows uncontrollably (observed 160K+ tokens), forcing manual compact command.

Proposed Solution

Add configuration for automatic compression:

{
  "session": {
    "maxContextTokens": 120000,
    "autoCompactThreshold": 0.75
  }
}

When context reaches 75% of max, auto-trigger compact to ~40-50% threshold.

Why

• Openclaw has similar auto-compact feature (works well)
• Qwen3.5-122B has 128K context limit, 75% (96K) is safe operational threshold
• Current manual workflow is error-prone for long debugging sessions

Impact

• Preserve session continuity without manual intervention
• Prevent GPU memory overload during long tasks