/compact custom instructions placeholder is misleading

Problem

The /compact command shows placeholder text: <optional custom summarization instructions> — implying you can control what gets compacted.

In practice, /compact always summarizes the entire conversation. The custom instructions only influence what the summary prioritizes, not what gets compacted vs. preserved.

Example

I had a session using ~239k tokens, mostly from large Bash tool results. I ran:

/compact just do this Bash results using 239.1k tokens (24%) → save ~119.5k

Expected: Only the Bash results would be compacted, preserving the rest of the conversation.

Actual: The entire conversation was compacted into a summary. Everything was replaced — not just the Bash output.

Why this matters

• The placeholder text reads like a filter/selector, not a priority hint
• Users trying to learn the tool get burned and lose important conversation context
• There's no undo — once compacted, the original conversation is gone

Suggestion

Either:

1. Make selective compaction work — let users target specific content types (tool results, bash output, etc.)
2. Change the placeholder text to something honest like <optional hints for what the summary should prioritize> so users know what they're getting