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>

SECURITY.md

@@ -0,0 +1,8 @@
+# Security policy
+
+If you find a security problem in Noo, please email
+[hello@ayushya.dev](mailto:hello@ayushya.dev) instead of opening a public
+issue. Include steps to reproduce and the version (`pubspec.yaml`). I'll
+acknowledge reports as soon as I can and credit you in the fix if you'd like.
+
+Never include real server URLs, credentials, or personal files in a report.