docs: add read-only mode specification, architecture, and ADR

- Specify OPENPROJECT_READ_ONLY and --read-only in docs/ARCHITECTURE.md

- Add ADR-011 in docs/DECISIONS.md for tool manifest filtering and execution guards

- Update docs/TODO.md with read-only implementation tasks

- Update README.md, AGENTS.md, and .env.example with read-only configuration reference

README.md

@@ -126,10 +126,23 @@ The MCP server accepts configuration through environment variables or a `.env` f
 | :--- | :--- | :--- |
 | `OPENPROJECT_BASE_URL` | Base URL of the OpenProject instance | `http://localhost:8080` |
 | `OPENPROJECT_API_KEY` | Personal API token (created under My Account > Access tokens) | - |
+| `OPENPROJECT_READ_ONLY` | Run server in read-only mode (`true` / `false` or `--read-only`) | `false` |
 | `PORT` | Local port for the Docker Compose web container | `8080` |
 | `TAG` | OpenProject container image tag | `17-slim` |
 | `POSTGRES_VERSION` | PostgreSQL container image tag | `17` |
 
+### Read-Only Mode
+
+To ensure AI assistants cannot make any modifications to your OpenProject instance, enable read-only mode:
+
+```bash
+OPENPROJECT_READ_ONLY=true bun run src/index.ts
+# or
+bun run src/index.ts --read-only
+```
+
+When active, write tools are omitted from the tool manifest and blocked by execution guards.
+
 ---
 
 ## Development Roadmap