[codex] Trim redundant CI build steps

[onevcat]

Summary

• remove the standalone CI make build-app step because xcodebuild test already builds the app/test host
• make make test self-contained by embedding the debug CLI before app tests
• keep make build-app available for local developer builds
• make ensure-ghostty fail immediately if the GhosttyKit rebuild fails

Why

Recent test workflow runs spend most of their time in a separate app build followed by make test. The test step performs its own Xcode build, so the standalone build duplicates work in CI.

Sample run timings inspected:

• 26089703494: make build-app 3m34s, make test 2m04s, total job 7m43s
• 26067525837: make build-app 2m10s, make test 1m28s, total job 5m18s
• 26041056706: make build-app 3m17s, make test 1m39s, total job 7m24s

Validation

Local validation was run without triggering remote CI repeatedly:

• removed generated Resources/prowl-cli/prowl
• cleared ~/Library/Developer/Xcode/DerivedData/supacode-*
• make lint
• DEVELOPER_DIR=/Applications/Xcode-26.4.0.app/Contents/Developer make test
• make test-cli-smoke
• make test-cli-integration
• git diff --check

[onevcat]

Closing this draft in favor of #307, which keeps the existing build step and parallelizes the test steps instead.