feat: build_settings.py — xcconfig / scheme / test plan inspect

[conorluddy]

Problem

CI debugging today is xcodebuild -showBuildSettings | grep ... in a million terminals. A structured introspector saves hours per CI incident.

Sketch

python scripts/build_settings.py --project MyApp.xcodeproj --scheme MyApp --effective
python scripts/build_settings.py --project MyApp.xcodeproj --schemes
python scripts/build_settings.py --project MyApp.xcodeproj --test-plans
python scripts/build_settings.py --project MyApp.xcodeproj --diff Debug Release

Wraps xcodebuild -list and -showBuildSettings, parses key=value, supports scheme/config diffing.

Reuses

• xcode/builder.py subprocess patterns
• common/cache_utils.ProgressiveCache for full setting dumps (these can be massive)

Open questions

• Diff format readability for hundreds of settings — collapsed by default, expanded with --verbose.
• Whether to support xcconfig file resolution.

Definition of done

• --effective, --schemes, --test-plans, --diff all working
• Output is grep-friendly in default mode
• --json, --help, SKILL.md updated

Part of the v2.0 coverage expansion epic.