Perf: loading, FRE, ACP connection, and error-detection speed

### Description of the new feature

Tracking issue for four speed items. Each needs a measurement first, then a target:

- Loading speed (app cold start to usable prompt)
- FRE speed (time to get through first-run and into the terminal)
- ACP connection speed (time from pane open to agent ready)
- Auto error detection speed (time from failed command to card appearing)

None of these have numbers attached today, so the first task is instrumenting each path and establishing a baseline. Sub-issues can be split out once we know where the time actually goes.

### Proposed technical implementation details

Related reports that may share a root cause: #386 (stuck on loading), #263 (terminal hang), #500 (Copilot ACP PreToolUse hook latency, closed).