docs: add agent guidelines, architecture specs, and decision records

- Add AGENTS.md defining coding standards, security, and workflows

- Add GEMINI.md and CLAUDE.md referencing AGENTS.md

- Add docs/ARCHITECTURE.md detailing MCP server design and OpenProject API integration

- Add docs/DECISIONS.md with initial ADRs (stack, transport, auth, scoping)

- Add .gitignore for Node.js and secrets protection

GEMINI.md

@@ -0,0 +1,5 @@
+# Gemini Instructions for openproject-mcp
+
+Please refer to [AGENTS.md](./AGENTS.md) for all project guidelines, coding conventions, architectural standards, and workflow instructions.
+
+All agents and assistants operating in this repository must follow the instructions defined in `AGENTS.md`.