fix: correct infrahub-mcp Dockerfile and README (#7)

## What changed

Fixes the build failure caused by wrong upstream repo URL and missing default branch.

### Dockerfile
- **Repo URL**: `opsmill/infrahub-mcp` (was `opsmill/infrahub-mcp-server`)
- **Default branch**: clone without `--branch` when `INFRAHUB_MCP_VERSION` is unset, so the upstream default branch is used automatically
- **Secrets warning**: removed `ENV INFRAHUB_API_TOKEN=""` — the token should be passed at runtime via `-e`, not baked into the image
- **Comments**: rewritten in English

### README
- Fixed upstream repo link
- Clarified that `INFRAHUB_API_TOKEN` is required at runtime

Reviewed-on: #7

images/infrahub-mcp/README.md

@@ -1,16 +1,16 @@
 # infrahub-mcp
 
-Docker image for the Infrahub MCP server ([OpsMill](https://github.com/opsmill/infrahub-mcp-server)).
+Docker image for the Infrahub MCP server ([OpsMill](https://github.com/opsmill/infrahub-mcp)).
 
-Packages the infrahub-mcp-server into a ready-to-use container,
-running the server in SSE mode on port 8001.
+Packages the infrahub-mcp server into a ready-to-use container,
+running in SSE mode on port 8001.
 
 ## Environment variables
 
 | Variable             | Description                      | Default                  |
 |----------------------|----------------------------------|--------------------------|
 | `INFRAHUB_ADDRESS`   | Infrahub instance URL            | `http://localhost:8000`  |
-| `INFRAHUB_API_TOKEN` | Infrahub API token               | (empty)                  |
+| `INFRAHUB_API_TOKEN` | Infrahub API token               | *(required at runtime)*  |
 | `MCP_HOST`           | Listen address                   | `0.0.0.0`                |
 | `MCP_PORT`           | Listen port                      | `8001`                   |