TUI crashes when opening a session that references a removed subagent

[legion-implementer]

What happens

When the TUI auto-syncs a session that references a subagent task whose own session has since been removed (deleted, cleaned up, or never persisted), the unhandled rejection from the 404 propagates and renders an error toast — or in some cases prevents the Task message from rendering at all.

Where

packages/opencode/src/cli/cmd/tui/routes/session/index.tsx — the auto-sync call in the Task message component fires sync.session.sync(props.metadata.sessionId) without handling the rejection.

Why this happens

Subagent sessions are ephemeral by design — they get GC'd, deleted, or never persisted depending on the agent's lifecycle. The parent session's Task message still references the subagent ID. When the TUI tries to fetch that ID, the backend returns 404.

Expected

The Task message renders without the subagent message preview. No error toast.

Repro

1. Open a session that used the Task tool to delegate to a subagent
2. Remove the subagent session (or let it GC)
3. Reopen the parent session in the TUI
4. Observe the error toast / missing render

Happy to submit a PR — fix is a one-line .catch(() => {}) plus a comment explaining the reasoning.

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [Bug]: Renderer crashes with TypeError: Failed to fetch in fetchMessages when background subagent completes #27292: Renderer crashes with TypeError: Failed to fetch in fetchMessages when a background subagent completes — same root cause (child session cleaned up/unavailable before UI fetch resolves), same general fix direction (guard the fetch against missing sessions).

[sjawhar]

Closing as duplicate of #27292 and #26871 (both describe the same underlying auto-sync 404 → unhandled rejection bug). The fix is already in flight as #26944. Apologies for the noise — I missed these in my initial search.