Session traces lost after crash — no crash handlers flush traces to disk

## Problem

When altimate-code crashes (TUI mode), session traces are lost. Users starting a new session cannot find logs from the previous crash.

**Root cause:** The TUI worker holds all trace data in memory. Bun Workers don't receive OS signals (SIGINT/SIGTERM/SIGHUP), so the worker dies silently when the main thread exits — no crash handlers flush traces to disk.

**Upstream context:** sst/opencode has 60+ open crash-related issues including:
- sst/opencode#19023 — Sessions permanently stuck after crash (no startup recovery)
- sst/opencode#14291 — Session not saved on unexpected termination
- sst/opencode#12767 — Missing SIGHUP handler causes orphaned processes

## Expected Behavior

Trace files should be persisted to `~/.local/share/altimate-code/traces/` with `status: "crashed"` when the process exits unexpectedly.

## Reported in

AltimateAI/altimate-code#588