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)

tabatago-swift/project.yml

@@ -30,7 +30,7 @@ packages:
     from: "5.0.0"
   PostHog:
     url: https://github.com/PostHog/posthog-ios
-    exactVersion: "3.62.0"
+    revision: "3.62.0"
 
 targets:
   TabataGo: