Chore: Add export-ignore to reduce SPM download size

- Mark non-essential files/dirs with export-ignore in .gitattributes
- Source archives (GitHub releases, package registries) now exclude:
  .claude/, .cursor/, .github/, .serena/, .vscode/, project-template/,
  AGENTS.md, CONTRIBUTING.md, WHATS-NEXT.md, to-dos.md, gource-swift.sh,
  .gource.conf, .windsurfrules, .swift-format, .swiftlint.yml
- Files remain tracked in git (history preserved)
- SPM consumers only receive Sources/, Tests/, Package.swift, README, LICENSE

.gitattributes

@@ -1 +1,19 @@
 docs/** linguist-documentation=false
+
+# Exclude non-essential files from source archives (git archive / GitHub releases).
+# SPM consumers only need Sources/, Tests/, Package.swift, README.md, and LICENSE.
+.claude/           export-ignore
+.cursor/           export-ignore
+.github/           export-ignore
+.serena/           export-ignore
+.vscode/           export-ignore
+project-template/  export-ignore
+AGENTS.md          export-ignore
+CONTRIBUTING.md    export-ignore
+WHATS-NEXT.md      export-ignore
+to-dos.md          export-ignore
+gource-swift.sh    export-ignore
+.gource.conf       export-ignore
+.windsurfrules     export-ignore
+.swift-format      export-ignore
+.swiftlint.yml     export-ignore