tui: stale subagent sessions cause SessionNotFoundError unhandled rejection bursts #47516

Summary

Multiple TUI clients spam SessionNotFoundError unhandled rejections for the same stale subagent session IDs, and concurrent session.json writes fail with Windows EPERM rename errors.

Environment

• opencode version: v0.0.0-beta-19151
• OS: Microsoft Windows NT 10.0.26200.0 (win32 x64)
• Terminal: Unavailable: terminal environment variables are not set
• Shell: C:\Windows\system32\cmd.exe
• Install/channel: beta
• Active plugins: local file openrouter-retry-guard.ts under ~/.config/opencode/plugin/; no plugin entries in ~/.config/opencode/opencode.json (provider orproxy configured)

Reproduction

1. Open several opencode TUI clients with no args against the same background service.
2. View a session containing Task subagent tool parts whose metadata.sessionId points to sessions absent from session_v2 (deleted, never projected, or from another directory).
3. Each client calls sync.session.sync(subagentSessionID), which uses throwOnError: true and throws Session not found: ses_....
4. The Task component fires this as void sync.session.sync(sessionID) with no catch, producing one unhandled rejection per client per stale session.
5. At the same time, concurrent session.json pinned-state writes collide on Windows rename.

Expected Behavior

Stale/missing subagent sessions should be pruned/ignored quietly: no unhandled rejections, no error burst across clients. Local session.json writes should tolerate concurrent writers.

Actual Behavior

Burst at 2026-09-05T11:11:49Z, one per CLI run ID per stale session:

unhandled rejection cause._tag=SessionNotFoundError cause.sessionID=ses_f8ebc4acdffeZcKq6Tl4hKag8F cause.message="Session not found: ses_f8ebc4acdffeZcKq6Tl4hKag8F" role=cli

Server returns 404 for the same IDs, e.g. GET /api/session/ses_f8ebc4acdffeZcKq6Tl4hKag8F 404, plus:

unhandled rejection cause="Error: EPERM: operation not permitted, rename '...\\session.json.<pid>.<uuid>.tmp' -> '...\\session.json'" role=cli

Additional Context

• Frequency: reproducible burst whenever stale subagent IDs are rendered; seen for ses_f8ebc4acdffeZcKq6Tl4hKag8F, ses_f8ec43de4ffe0hEYUORBF4cIXU, ses_f8ec7b520ffe70bI7626Titf1k, ses_f8eccfda2ffeLJbQHoTfomFavs.
• Checked opencode.db: session_v2 had 5 rows; the failing IDs were absent.
• Suspected read-path gap (creation-race fixes do not cover reads): packages/tui/src/context/sync.tsx sync() throws on 404; packages/tui/src/routes/session/index.tsx Task onMount uses void without catch.
• Local work in progress in fork checkout (not yet installed): tolerate 404 in sync.session.sync + V2 data.session.refresh, swallow Task preview sync errors, retry writeJsonAtomic rename on EPERM/EBUSY/EACCES.
• Workaround tried: none; restarting service/CLI does not help because the stale IDs are re-requested.