Auto-compaction starts without warning, no opportunity to use /compact manually

Problem

Auto-compaction fires silently mid-session with no visible warning. The user has no opportunity to:

• Know compaction is about to start
• Run /compact manually with a custom summary prompt to control what gets preserved
• Save important session context before it is summarized away

The first sign anything happened is that part of the conversation history is already gone.

Impact

• Critical context from the start of a long session can be permanently lost
• Scheduled background tasks (e.g. ScheduleWakeup) that were registered in the compacted portion are silently dropped
• The compaction summary may not capture domain-specific context that the user would have included in a manual /compact prompt

Expected behaviour

• Show a warning (e.g. "Context is approaching the limit — compaction will start in X messages") so the user can intervene
• OR pause before compacting and prompt the user: "Context limit approaching. Run /compact [prompt] now, or press Enter to auto-compact."

Environment

Claude Code VSCode extension, macOS, claude-sonnet-4-6