[UX] Design legislative_impact_map relation workflow

[StatPan]

Parent: #60
Depends on: #62, #63, #65

Goal

Add an AssemblyMCP-native impact map workflow that returns graph-ready relationships between bills, topics, members, committees, meetings, votes, and reports.

This is inspired by KoreaLawMCP's impact_map, but the National Assembly domain should use legislative relations rather than legal article citations.

Scope

• Define node and edge schema before implementation.
• Support at least bill and topic targets in the first implementation.
• Include evidence for every edge: source tool, source ID, and relevant fields.
• Optionally include Mermaid graph text, but structured nodes/edges are the primary contract.

Acceptance Criteria

• Relation schema is reviewed before broad implementation.
• First implementation supports bill and topic targets.
• Every edge has evidence metadata.
• Output remains useful without graph rendering.
• Tests cover relation extraction and empty/partial graph cases.