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

CLAUDE.md

@@ -1,5 +1,33 @@
 # Claude Instructions for openproject-mcp
 
-Please refer to [AGENTS.md](./AGENTS.md) for all project guidelines, coding conventions, architectural standards, and workflow instructions.
+Please refer to [AGENTS.md](./AGENTS.md) for primary 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`.
+---
+
+## Runtime & Tooling (Bun)
+
+Default to using Bun instead of Node.js:
+
+- Use `bun <file>` instead of `node <file>` or `ts-node <file>`
+- Use `bun test` instead of `jest` or `vitest`
+- Use `bun install` / `bun add` instead of `npm install` or `pnpm install`
+- Use `bun run <script>` instead of `npm run <script>`
+- Use `bunx <package> <command>` instead of `npx <package> <command>`
+- Bun automatically loads `.env` and `.env.local`, so don't use `dotenv`.
+- Prefer `Bun.file` over `node:fs` for reading and writing files.
+
+## Testing
+
+Use `bun test` to run tests:
+
+```ts
+import { test, expect, describe } from "bun:test";
+
+describe("OpenProject Client", () => {
+  test("authenticates via Basic Auth", () => {
+    expect(1).toBe(1);
+  });
+});
+```
+
+For full details on project architecture and decisions, see [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) and [docs/DECISIONS.md](docs/DECISIONS.md).