Conversation lifecycle tools: guided compaction and cache-aware sessions

Provenance stamp: This issue was triaged on June 13, 2026 as part of the older-than-two-month GitHub issue triage exercise.

Source issues: #402, #417

## Summary

Long agent sessions create two related problems: users need to preserve the right context during compaction, and long review pauses can interact badly with provider prompt-cache behavior. These issues should be considered together as conversation lifecycle tooling.

## What the source issues requested

- #402 requested an interactive `/plannotator-compact` workflow where the current conversation is rendered in Plannotator and users can mark sections to keep, drop, or summarize before compaction.
- #417 requested an automatic or configurable cache keep-alive mechanism that warns before prompt cache expiry and can send a no-op message to renew the cache.

## Triage determination

Both requests are about keeping long sessions useful while avoiding context loss or expensive cache misses. The compaction request is the safer product anchor. Cache keep-alive behavior should be treated carefully because automatic dummy messages can surprise users and may be runtime-specific.

## Acceptance criteria

- Current conversation or session logs can be rendered as reviewable blocks.
- Users can mark conversation sections as keep, drop, or summarize.
- The tool can return structured compaction directives to the agent.
- Large tool outputs are collapsed or summarized enough to make review practical.
- Cache keep-alive behavior is opt-in, visible, and runtime-specific.
- Automatic no-op messages are avoided unless the runtime can make the behavior explicit and reversible.
- The old source issues can be closed once this issue captures the long-session lifecycle requirements.