OpenCode chat: subagents are not surfaced like Claude Code's #893

Problem

Claude Code panes surface their session's subagents as background work: running count in the message-box button and sidebar badge, an ended-subagent card with its answer, BG turn state, and lost detection (since 0.4.4, #527/#534; guide). An OpenCode pane gets none of it, even though OpenCode 2 records each subagent as a child session in the same store the chat reads.

Evidence (v0.4.8, OpenCode v2.0.27)

• The reader is bound to the one session herdr reports (herdr:opencode) and every page query filters WHERE session_id = ? — opencode.ts#L72, #L330. Child sessions are never read.
• A subagent is session_v2.parent_id plus its own session_message rows (own seq space). Confirmed in a live store: 1,114 child sessions, 22,016 child messages.
• The parent's only record is the subagent tool part: {agent, description, prompt}, state.status running, answer wrapped in raw <subagent …> text; a background call gets a launch ack and later a synthetic wake row (source: "subagent") drawn as a plain notice.
• No test or doc mentions OpenCode subagents.

Acceptance

• An OpenCode pane's subagents counted and listed where a Claude pane's are: button count, sidebar badge, newest ten ended in the last day.
• An ended subagent leaves a card in the chat with its answer.
• A turn that ends while its subagents run reads BG, the finish alerted once; a subagent whose pane no longer runs that session reads as lost.