Add support for instructions field in initialize response

[douweh]

The MCP specification (2024-11-05+) supports an instructions field in the server's initialize response. This allows servers to
provide guidelines to clients about how to use the server effectively.

From the spec:
The server MAY include an instructions field containing human-readable instructions for the client.

Use case: I'm building a time tracking MCP server and want to provide guidelines like:

• Round times to 10-minute increments
• Minimize entries by combining related work
• Use descriptive labels

Suggested API:
use Anubis.Server,
name: "my-server",
version: "1.0.0",
capabilities: [:tools],
instructions: """
Guidelines for using this server:
- Guideline 1
- Guideline 2
"""