/compact -n <lines>: partial compaction retaining recent conversation

Problem

When context approaches limits mid-session, the only option is full compaction which summarizes ALL prior messages. This loses:

• Recent system state (tool outputs, file changes, running processes)
• Active debugging context that was just established
• Multi-step workflow progress that hasn't been committed yet

Users often notice context pressure while actively working, but can't compact without losing the last few exchanges that contain critical state.

Proposed Solution

/compact -n 5000 — compact messages BEFORE the last N lines, retain the last N lines verbatim.

Behavior:

1. Lines 1 to (total - N): summarized into compaction summary
2. Lines (total - N + 1) to end: kept as-is
3. Result: summary + recent conversation, within context budget

Use Cases

• Long debugging session: keep recent errors/fixes, compact old exploration
• Multi-agent orchestration: keep recent agent results, compact earlier rounds
• Extended pair programming: keep current task context, compact yesterday's work

Prior Art

The existing /compact already produces a summary and truncates. This extends it with a retention window, similar to how log rotation keeps recent entries while archiving old ones.