refactor: remove unused petgraph dependency from agent crates (#390)

Removes petgraph from terraphim_goal_alignment, terraphim_task_decomposition,
and terraphim_agent_registry. Analysis confirmed zero actual usage in source
code - all graph functionality is provided by terraphim_rolegraph::RoleGraph.

Architectural rationale:
- RoleGraph provides domain-specific graph optimized for semantic search
- Aho-Corasick automata integration for text matching
- Custom DFS for path connectivity (optimized for k<=8 nodes)
- Integrated document ranking built into graph traversal

Benefits:
- Reduced compilation time and dependency surface
- Clearer architectural intent
- Removed transitive dependencies (fixedbitset, etc.)

Generated with Terraphim AI

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: AlexMikhalev