[FEATURE]: Add a config option to hide the compaction divider/marker in the TUI

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Describe the enhancement you want to request

When auto-compaction runs, the TUI renders a "Session compacted" divider in the message stream. As far as I can tell there's no config option to suppress it — I checked the full config schema and tui.json for something like a locale override or an env var that would hide the label, and found nothing.

This gets noisy fast when running against local/self-hosted models (llama.cpp, Ollama via @ai-sdk/openai-compatible), where compaction tends to fire much more often than it does against large cloud models — every compaction adds a visible divider that clutters the session view. It's a separate concern from the compaction reliability issues already tracked for local backends (#22792, #15533, #27924), but it compounds them: when compaction already fires more often and sometimes loops, having no way to quiet the visual noise makes it worse to work through.

Proposed solution: a config option under the existing compaction schema block (e.g. compaction.hideMarker, boolean, default false) that suppresses rendering of the divider while leaving the compaction event itself — summary generation, token accounting — unaffected.

Alternatives considered:

• Patching the bundled opencode binary to no-op the divider render — works, but doesn't survive opencode upgrade and would need a custom reapplication script on every update, which isn't something most users want to maintain.
• Living with the divider — acceptable, just noisy, especially in longer local-model sessions.