Detect cycles in service dependencies

[riken127]

Title

Add cycle detection for depends_on graph

Description

Currently depends_on validates existence and duplicates, but does not detect cycles.
Example invalid case:

service A depends_on B
service B depends_on A

This should be rejected during semantic validation.

Acceptance Criteria

• Graph-based cycle detection implemented
• Clear error messages
• Covered by tests

Notes

Simple DFS or Kahn’s algorithm is enoug