Prepare repository for public release

Add PolyForm Shield license, contributing and security docs, PR and commit
templates; rewrite README; ignore local Claude and signing files.

Co-Authored-By: Claude Code <noreply@anthropic.com>

pubspec.yaml

@@ -1,5 +1,5 @@
 name: noo
-description: "A new Flutter project."
+description: "Noo - a fast, native Nextcloud client built with Flutter."
 # The following line prevents the package from being accidentally published to
 # pub.dev using `flutter pub publish`. This is preferred for private packages.
 publish_to: 'none' # Remove this line if you wish to publish to pub.dev