docs: adapt project guidelines, architecture, and ADRs for Bun

- Update AGENTS.md, CLAUDE.md, and GEMINI.md with Bun workflow rules and bun:test

- Update docs/ARCHITECTURE.md with Bun runtime layer and lockfile

- Add ADR-010 in docs/DECISIONS.md superseding ADR-001 with Bun adoption

- Update README.md with Bun prerequisites, installation, and run instructions

README.md

@@ -32,7 +32,7 @@ A complete OpenProject 17 environment with PostgreSQL 17 and Memcached is includ
 ### Prerequisites
 
 - Docker and Docker Compose (v2+)
-- Node.js (>= 18) and npm
+- [Bun](https://bun.sh) (>= 1.2 / 1.3)
 
 ### 1. Start the Environment & Seed Data
 
@@ -89,6 +89,34 @@ docker compose down -v
 
 ---
 
+## MCP Server Development (Bun)
+
+Install project dependencies:
+
+```bash
+bun install
+```
+
+Start the MCP server over stdio:
+
+```bash
+bun run src/index.ts
+```
+
+Run test suite:
+
+```bash
+bun test
+```
+
+Type-check:
+
+```bash
+bun run typecheck
+```
+
+---
+
 ## Configuration
 
 The MCP server accepts configuration through environment variables or a `.env` file: