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

[KrizTahimic]

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

• [BUG] Haiku 4.5 incorrectly limited to 8192 output tokens instead of 64K #9621 (Haiku token limit bug affects compact)
• [ENHANCEMENT] Use Claude Haiku 4.5 for commit-commands plugin #9623 (Haiku for simpler tasks)
• Automatic Model change for specific tasks and in middle of workflows #5924 (Task-specific model switching)

[github-actions]

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Found 3 possible duplicate issues:

1. [FEATURE] Allow model selection in slash command frontmatter #7795
2. Feature Request: Add model selection support for custom commands (similar to subagents) #4937
3. [ENHANCEMENT] Use Claude Haiku 4.5 for commit-commands plugin #9623

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[github-actions]

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