Add 5 new specialized subagents and update README

- Add architect-reviewer for code architecture reviews
- Add context-manager for managing context across agents
- Add dx-optimizer for developer experience improvements
- Add incident-responder for production incident handling
- Add prompt-engineer for LLM prompt optimization
- Add .gitignore file
- Update README.md with new subagents and correct count (28 total)

.gitignore

@@ -0,0 +1,22 @@
+# Folder view configuration files
+.DS_Store
+Desktop.ini
+
+# Thumbnail cache files
+._*
+Thumbs.db
+
+# Files that might appear on external disks
+.Spotlight-V100
+.Trashes
+
+# Compiled Python files
+*.pyc
+
+# Compiled C++ files
+*.out
+
+# Application specific files
+venv
+node_modules
+.sass-cache