FRE-4661: Implement ShieldAI PR workflow recommendations

- Add Gitea PR template with code review checklist
- Add CODEOWNERS file for review assignment policy
- Update shieldai-workflow.md with implemented workflow
- Add branch-protection-rules.yaml for gt/master protection

Co-Authored-By: Paperclip <noreply@paperclip.ing>

CODEOWNERS

@@ -0,0 +1,18 @@
+# CODEOWNERS
+# This file defines ownership of directories and files
+# Rules are evaluated from top to bottom; first match wins
+
+# Senior Engineer owns all TypeScript source files
+*.ts     @senior-engineer
+*.tsx    @senior-engineer
+
+# Founding Engineer owns architecture and configuration
+*.config.*  @founding-engineer
+*.json      @founding-engineer
+
+# Security Reviewer owns security-critical paths
+**/auth/**       @security-reviewer
+**/middleware/**  @security-reviewer
+
+# Code Reviewer reviews all Pull Requests
+*      @code-reviewer