Feature hasn't been suggested before.

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Describe the enhancement you want to request

Problem

When auto-compaction fires (compaction.auto: true), it runs synchronously in the session loop, freezing the prompt input until it completes. On slow models or large contexts, this can lock the user out for minutes. The TUI becomes unresponsive and you can't even type.

Proposed behavior

• Compaction runs as a background job — user keeps chatting uninterrupted
• New messages sent during compaction get appended to the uncompacted history
• Once compaction finishes, the old history gets replaced by the compacted summary
• A toast or indicator shows "Compacting…" while it runs
• On completion, emit session.compacted and swap in the compacted context

Existing foundation

SDK already defines CompactionPart and EventSessionCompacted — the event plumbing is there, only the execution path needs to become non-blocking.

Config

Could expose a toggle (default false for safety):

"compaction": {
  "auto": true,
  "background": true
}