Add names-only output to azd app graph

[jongio]

Scripts sometimes only need the graph node names after filters are applied. A names-only view avoids parsing diagram or JSON output for simple loops.

Acceptance criteria:

• azd app graph --output names prints one node name per line.
• Names are sorted by graph level, then alphabetically within each level.
• --focus and --services-only affect the names list.
• Tests cover service-only filtering and empty graph behavior.