[Feature Request] Allow /compact to accept custom summarization instructions

What feature would you like to see?

Description

• Background: /compact currently uses a fixed prompt, so users cannot steer summarization (e.g., “code only”, “avoid personal info”). Claude Code supports /compact , and we have the same need.
• Problem: Manual /compact calls can’t accept a hint, so the summary may not fit the scenario. Automatic compaction should keep the default behavior.
• Proposal:
  1. Let /compact accept an optional hint /compact and pass it through to SimpleCompaction.
  2. In SimpleCompaction, append User instruction: after the default prompts.COMPACT; if no hint is provided, keep current behavior.
  3. Update help text and add a test covering custom instruction injection.
• Impact: Defaults unchanged; only user-invoked /compact with a hint alters the compaction prompt.
• Validation: Add/adjust tests; manual check with /compact summarize code only to see the summary change.

Additional information

No response

Start by locating the /compact command entry point and the SimpleCompaction implementation, then find the existing help text and compaction tests. Add coverage for injecting an optional hint after prompts.COMPACT while preserving the no-hint behavior. Done means /compact accepts a hint, automatic compaction remains unchanged, and the updated help text and tests pass.