[FEATURE]: Silent/background compaction — don't stream summary output to the terminal #13033

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When auto-compaction triggers, the entire summarization process streams visibly into the chat window. The user has to sit and watch the summary text render token-by-token before they can continue working. This is disruptive and slow — especially since compaction uses the same primary model as the active session by default.

Current behavior:
The compaction summary streams into the chat window just like a normal assistant response. This interrupts the user's flow, is slow (uses the primary model), and adds visual noise — the summary is internal context for the AI, not something the human needs to read in real-time.

Expected behavior:
Compaction should run silently in the background. The user should see at most a brief indicator (e.g., "Compacting context..." → "Done"), not the full streamed output.

Suggestion:

• Stream compaction in the background without rendering output to the chat
• Show a minimal status indicator (spinner, progress bar, or one-liner like "Context compacted")
• Optionally, make this configurable (e.g., compaction.silent: true) for users who want to watch it

Not a duplicate of flagged issues:

• [FEATURE]: Configuration option to disable message summary generation #6228 is about disabling per-message summary generation (small summaries, not compaction)
• Feature: Configurable compaction threshold and model (global + per-model) #11930 is about configurable compaction threshold and model selection
• [FEATURE]:Session compaction includes already-summarized messages, wasting tokens #12479 is about token waste from re-summarizing already-summarized messages

This issue is specifically about the UX: compaction output should not stream visibly into the user's chat window.

Environment: opencode 0.0.0-dev-202602020741, macOS (desktop/web app)