Add D2 diagram output to `azd app graph`

[jongio]

azd app graph already exports text, json, markdown, mermaid, and dot. D2 (terrastruct) is a modern diagram language with clean auto-layout and good rendering tooling, and it is a natural next format for the dependency graph.

Add d2 as an --output value that renders the service dependency graph as a D2 diagram.

Acceptance criteria:

• azd app graph --output d2 prints valid D2.
• Nodes are service names; edges follow dependency direction.
• Works with --output-file and --focus like the other formats.
• Follows the existing one-function-per-format renderer structure.
• Unit tests cover the D2 output, including the focus case.
• Documented in the CLI reference.