fix(session): bound /btw side-question snapshot to recent messages

[YOMXXX]

Summary

Fixes #1855. /btw <question> is marketed as a "side question without context cost", but forkQuery (packages/opencode/src/tool/session.ts) passed the entire session history to buildPrefix, so the fork's prompt was the full session (observed 1M+ input tokens on ~300k-token sessions → empty answer / hang).

Fix: bound the side-question snapshot to the most recent MAX_FORK_QUERY_MESSAGES (12) messages, while guaranteeing the last user message stays in the window (buildLLMRequestPrefix dies without a user message — an uncatchable defect).

• sliceSideQuestionContext(msgs, max, mustInclude?) — new helper: normally the last max messages; extends to include the last matching element (the last user message) if it would otherwise be cut off.
• forkQuery passes (m) => m.info.role === "user" as the predicate.
• watermark (session boundary) and agent identity stay computed from the full session — only the snapshot content is bounded.
• Covers all side-ask paths (session.ask route + session ask tool op both call forkQuery).

The fork's prompt is [...forkCtx.inheritedMessages, ...ownNewModelMsgs] and does not re-pull history, so bounding inheritedMessages bounds the fork.

Test Plan

• New unit tests: sliceSideQuestionContext bounds to last max; keeps all under cap; preserves order; extends window to include the last user message when the tail has none; stays bounded when the tail already contains a user message.
• bun test test/tool/session-side-question.test.ts — 5 pass.
• bun typecheck — clean.