Make session cache GC target paths configurable

[zzj3720]

Follow-up from PR #60 review.

The Step 1 session cache GC uses hard-coded relative paths, including Cueboard-specific macOS build output such as frontend/macos/.build/DerivedData. This is acceptable for the initial cleanup PR, but future macOS/iOS repos may use different layouts.

Add configuration for session cache GC target paths, with safe defaults and platform guards. Consider:

• default built-in paths matching PR Add safe session cache GC dry run #60 behavior;
• optional env/config override or extension list;
• validation that configured paths remain relative and cannot escape the session workspace;
• tests for configured Darwin-only and cross-platform paths.

Related PR: #60