Background compaction consumes 1 premium request (VS Code does not)

### Describe the bug

Background compaction in Copilot CLI incorrectly consumes 1 premium request each time it triggers. The VS Code Copilot Chat extension performs the same compaction/summarization operation without consuming
a premium request.

Observations
- The VS Code extension handles compaction/summarization without consuming premium requests. It uses a caller-controlled boolean to mark requests as agent-initiated.
- The CLI's compaction appears to be incorrectly classified as a user-initiated request rather than an agent/background operation.
- Session naming (also a background operation in CLI) does NOT consume a premium request — only compaction does.
- This means long sessions with premium models consume significantly more premium requests than expected due to repeated compaction cycles.

### Affected version

Copilot CLI v1.0.4 and v1.0.5

### Steps to reproduce the behavior

1. Start a Copilot CLI session with a premium model (e.g., Claude Sonnet 4)
2. Have a long conversation until context reaches ~80% utilization
3. Send another message — this triggers background compaction
4. Check the premium request counter on the GitHub billing dashboard

### Expected behavior

Background compaction should be free, consistent with VS Code behavior.

### Additional context

_No response_