Graphify repository integration

Export repository dependencies and service relationships as graphs.

arka graphify . --format mermaid --output repo.mmd
arka graphify . --format dot --output repo.dot --render svg
arka graphify . --format json --output repo.json

Graphify uses Arka’s existing repository graph, so it remains local and read-only. DOT rendering requires Graphviz (dot) to be installed.