[Feature Request] Add compact() method for programmatic context compaction with custom instructions

Problem

The CLI supports /compact <custom_instructions> to compact conversation context with custom summarization instructions. However, the SDK provides no way to trigger this programmatically.

The current control protocol subtypes are: initialize, interrupt, set_permission_mode, set_model, rewind_files, mcp_status — there is no compact subtype.

The PreCompact hook only reads custom_instructions and trigger but cannot initiate compaction or modify the compaction instructions.

Proposed Solution

Add a compact() method to ClaudeSDKClient:

async with ClaudeSDKClient(options=options) as client:
    # ... long conversation ...
    
    # Compact with custom instructions
    await client.compact(
        instructions="Preserve all discovered vulnerabilities, open ports, and incomplete test tasks"
    )
    
    # Continue with compacted context
    await client.query("Continue to next phase")

Implementation Suggestion

Add a new control request subtype "compact":

{
  "type": "control_request",
  "request": {
    "subtype": "compact",
    "custom_instructions": "Preserve all vulnerability findings..."
  }
}

Use Cases

1. Long-running agent sessions: Compact context periodically while preserving critical domain-specific information
2. Phase transitions: When switching task phases, compact with phase-specific instructions (e.g., "keep recon findings, discard exploration attempts")
3. Token budget management: Proactively compact before hitting token limits with domain-aware summarization
4. Automated pipelines: Integration into CI/CD or automated testing workflows where manual /compact is not possible

Environment

• SDK Version: 0.1.33
• CLI Version: 2.1.37