Feature idea: expose an idle + explicit-span compaction entry point (idle compactRegion)

[DerekZhangbj]

Hi — thanks for open-sourcing this; "everything is a plugin" has made it very pleasant to build on.

The gap

Compaction currently has two halves, and neither alone can express "compact exactly this finished span while the agent is idle":

• compactNow(agent, signal, sourceCommandId?) — idle-friendly, but the span is chosen by selectCompactableRange, which only takes a prefix (and keeps the priced tail).
• compactRegion(start, end, agent, signal?) — takes an explicit span, but requires an open turn (owner: "current-turn", stability: "whole-surface").
• /compact — still USAGE = 'Usage: /compact (no arguments)'.

GUI surfaces (composer buttons, plugin panels, slash commands) act while the agent is idle, so they can't call compactRegion, and compactNow can't target the span the user picked. The span people usually want to drop is a recent side branch (a debugging detour, a dead end), not the oldest prefix.

What a community plugin had to do

I maintain dsh-compaction-selective (https://github.com/DerekZhangbj/dsh-compaction-selective): the user names the span, a confirmation card is shown, then the span is folded. For the idle path it patches dsh-compaction-basic with ~15 lines that mirror compactNow's manual path:

compactRange(start, end, agent, signal, sourceCommandId) {
  return agent.runMaintenance(async (agentSignal) => {
    const operationSignal = AbortSignal.any([agentSignal, signal]);
    return await compactSurfaceRegion(this.regionDependencies(), agent.session, start, end, agent,
      { owner: null, stability: "selected-span", flush: () => this.ctx.sessions.flush(agent.session) },
      operationSignal);
  });
}

That works on a CLI install (verified on 0.2.0-rc.2, 0.2.1-rc.2).

Why an official entry point would matter more than a patch

The desktop app bundles the core inside app.asar. Third-party plugins cannot patch it, and even on the CLI a local patch is wiped by every upgrade. So for the desktop app the plugin has to degrade: the panel disables the actions and tells the user to use the conversation form instead. Every desktop user of any plugin that wants idle-span compaction hits the same wall.

The ask

Expose a supported entry point for idle + explicit span compaction — e.g. a compactRange(start, end, agent, signal?, sourceCommandId?) method on CompactionEngine (or an option on compactRegion such as owner: null). If there's already a sanctioned way to do this that I missed, pointing me at it would be just as good.

I'm happy to open a PR — I read CONTRIBUTING.md and understand external PRs are paused at the moment, so I'm posting here first. No expectations on timing; I mainly want to know whether this is a direction you'd accept, and what shape you'd prefer.

Context if useful: my compatibility notes with the exact tags/SHAs inspected are in COMPATIBILITY-AUDIT.md in the repo above.