fix: permanently fix PostHog module emission error

- Change PostHog from exactVersion to revision (tag 3.62.0) in project.yml
- Add deep cache clean step before xcodegen in CI workflow
- Add ONLY_ACTIVE_ARCH=NO to xcodebuild command
- Wrap PostHog usage in #if canImport(PostHog) for graceful degradation
- Add Package.resolved, .build/, DerivedData/ to .gitignore
- Remove stale SPM caches between CI runs (DerivedData, swiftpm caches, act cache)

.gitignore

@@ -58,3 +58,8 @@ Config/Secrets.xcconfig
 _Users_*
 swift-generated-sources/
 tabatago-swift/build/
+
+# Swift Package Manager
+Package.resolved
+.build/
+DerivedData/