[codex] Run CI test steps in parallel

[onevcat]

Summary

• keep the explicit CI app build step
• add a test-app make target for app/unit tests only
• run app tests, CLI smoke, and CLI integration concurrently after build-app
• keep make test self-contained for local usage by embedding the debug CLI before test-app
• make ensure-ghostty fail immediately if the GhosttyKit rebuild fails

Why

A previous attempt to remove the standalone build-app step passed but did not speed up CI because make test absorbed the full build cost. This branch keeps build-app and only parallelizes the independent test commands that currently run after it.

Expected impact is modest but lower-risk: roughly the serial tail of CLI smoke + CLI integration, around 40-50 seconds on recent runs.

Validation

Local validation before pushing:

• make build-app
• parallel make test-app / make test-cli-smoke / make test-cli-integration using the same shell pattern as this workflow
• git diff --check