[FEATURE] Pre-compaction notice + visible record of what compaction dropped

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet — nearest matches: #34501 (closed; asked to preserve history visibility), #14230 (closed; general compaction critique), #82184 (open; about governance rules being dropped — different scope). No open request for pre-compaction notice + loss record specifically.
• [x] This is a single feature request (not multiple features)

Problem Statement

Auto-compaction is silent until after the fact: the "[Context compacted. Continuing from summary...]" notice arrives only once the loss has already happened, and there is no visible record of what was dropped. In long research/agent sessions this means neither the user nor the agent can tell which established facts, tool outputs, or decisions survived — the gap is only discovered later, when the agent re-derives or misremembers something that was previously settled. Teams end up building external state files (STATUS docs, ledgers) purely to compensate.

Proposed Solution

Two additions to compaction:

1. Advance notice: a brief pre-compaction signal (like the existing context-usage indicator, but at the compaction decision point) giving the user a chance to /compact <instructions> with intent, or to checkpoint state first.
2. Loss record: after compaction, an inspectable summary-of-the-summary — e.g. a /compact-log view listing which conversation spans were folded and which were preserved verbatim — so both user and agent can audit what the session no longer remembers.

Alternative Solutions

• CLAUDE.md compaction directives ("When compacting, always preserve...") help steer what is kept but provide no notice and no audit of what was lost.
• Manual /compact before critical junctures — requires the user to predict compaction timing.
• External state files maintained by the agent (our current workaround) — works, but re-implements memory outside the tool.

Priority

Medium - Would be very helpful