Prepare for pub.dev publishing with Gitea release pipeline

- Add README.md and CHANGELOG.md (required by pub.dev)
- Add .pubignore / ios/.pubignore to include xcframework in published package
- Add Gitea Actions workflow: builds xcframework and publishes on release
- Release tag must match pubspec version with no v prefix (e.g. "0.1.0")
- Requires PUB_TOKEN secret in Gitea repo settings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.pubignore

@@ -0,0 +1,19 @@
+# Git-specific
+.git/
+.gitignore
+
+# Build artifacts (but NOT TailscaleKit.xcframework — that ships with the package)
+.dart_tool/
+.packages
+.pub/
+build/
+
+# Go source and build tools (users don't need these)
+ios/Go/
+ios/_current_slice
+ios/.gitignore
+build_go.sh
+test/
+
+# Override ios/.gitignore — the xcframework MUST ship with the package
+!ios/TailscaleKit.xcframework/