Feature: Selective context compaction — allow discarding context before a checkpoint

Problem

When running a context-heavy task (e.g., a large PR review) mid-session after a long conversation, the compactor treats all prior context equally. It may summarize away the new, important context (like a PR diff being analyzed) just as readily as the older, irrelevant discussion that preceded it.

The only workaround today is to start a fresh session, which breaks workflow continuity.

Proposed Solution

Allow users (or tools) to set a context checkpoint — a marker that tells the compactor: "everything before this point can be aggressively dropped or heavily summarized; prioritize retaining everything after it."

Possible interfaces:

• Slash command: /checkpoint — marks the current point in conversation. When compaction triggers, context before the checkpoint is discarded first.
• Tool API: A mechanism for tools to signal "this is a fresh unit of work — prior context is low priority" when they start.
• Auto-checkpoint on skill invocation: Automatically treat skill boundaries as soft checkpoints.

Use Case

1. User has a long session — debugging, exploring code, chatting.
2. User invokes a context-heavy task that needs significant window for diffs, file reads, and analysis.
3. Today: compaction fires and may lose the task context. User has to start a new session.
4. With this feature: the invocation acts as a checkpoint, and the compactor preferentially drops pre-checkpoint conversation.

Additional Context

This would benefit any workflow where users switch tasks mid-session. The ability to say "I’m done with that topic, prioritize the new one" would make long sessions much more usable.

Related but distinct from existing compaction issues (#50888, #51207) which focus on when compaction fires — this is about what gets compacted.