fix(web): room timeline is flat and chronological; sub-threads are metadata (CL-8445)

[TheGreatAxios]

Summary

• readRoom now returns every turn oldest-first as a flat, top-level RoomMessage list — no turn is ever nested out of the main timeline based on In-Reply-To.
• RoomMessage.replies is replaced with parentMessageId (set only when the parent is a known message), plus a new ancestorChain helper the sub-thread panel uses to walk a turn's ancestor chain.
• workbench-room-page.tsx renders every turn inline in the main scroll; a per-row "Reply" action opens the sub-thread panel (via ancestorChain) to reply in context, same as before for person-initiated inReplyTo sends. Agent replies always show inline now, regardless of their In-Reply-To header.

Test plan

• bunx tsc -p apps/web --noEmit
• bun run fmt
• bun run lint

[linear-code]

CL-8445