Add architecture documentation and update gitignore

Add ARCHITECTURE.md (635 lines) documenting:
- Migration rationale and goals
- Go-based buildpack architecture
- Supply/finalize phase separation
- Extension system design
- Configuration management approach
- Comparison with Python implementation

Update .gitignore to exclude Go build artifacts and temporary files.

Author: ramonskie

.gitignore

@@ -20,3 +20,14 @@ DebugKit
 Dockerfile
 lib/PyYAML*
 lib/_yaml
+
+# Build artifacts
+*.test
+.bin/
+build/*.zip
+
+# Sensitive files
+AGENTS.md
+
+# Test artifacts
+test-verify*/