[FEATURE]: /btw — ask a quick side question without polluting session history

Requested feature

Add a /btw slash command (inspired by Claude Code's /btw) to ask a fast secondary question that does not become a permanent part of the conversation history used for context.

Why

There is no ephemeral question channel today. Side questions that aren't relevant to the task currently get added to the session, inflating the context window and the transcript.

Proposed behavior

• /btw <question> routes the prompt through the model but marks it non-durable / excluded from session history synthesis (packages/opencode/src/session).
• The answer is shown inline (or in a toast/dialog) but not stored as a first-class user+assistant message pair.
• Configurable via a flag so it can be persisted if desired.

Benefits

• Maintain a lean context window; keep tangential questions out of the record.
• Quick Q&A without creating noise in exported/compressed sessions.