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>

.gitignore

@@ -45,3 +45,14 @@ app.*.map.json
 /android/app/profile
 /android/app/release
 
+# Local Claude Code state
+.claude/worktrees/
+.claude/settings.local.json
+.claude/scheduled_tasks.lock
+
+# Signing / secrets (never commit)
+*.keystore
+*.jks
+key.properties
+.env
+.env.*