[FEATURE] Add compactModel setting to use cheaper model for /compact

Feature Request

Add a compactModel configuration option to use a different (cheaper/faster) model specifically for the /compact command.

Proposed Setting

In .claude/settings.json:

{
  "model": "opus",
  "compactModel": "haiku"
}

Rationale

• Compaction/summarization is a simpler task than complex reasoning or code generation
• Using Opus/Sonnet for compaction is unnecessarily expensive
• Haiku 4.5 is capable enough for summarization at 66% lower cost
• Similar to how opusplan uses different models for different tasks

Related Issues

• #9621 (Haiku token limit bug affects compact)
• #9623 (Haiku for simpler tasks)
• #5924 (Task-specific model switching)