[UX] Implement issue_brief workflow

[StatPan]

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

Goal

Add a workflow tool that answers: "What is happening in the National Assembly about this topic?"

This should be the first end-to-end AssemblyMCP workflow tool and should prefer useful synthesis over raw lists.

Scope

• Search related bills for a topic.
• Identify current committees from bill data.
• Include relevant meeting records when discoverable.
• Include legislative reports/news from existing report tooling.
• Include voting signal where available.
• Return follow-up tool suggestions.
• Use [PARTIAL] marker payloads when a section fails.

Acceptance Criteria

• Tool returns structured sections: topic, bills, committees, meetings, reports, voting_signal, follow_up, warnings.
• Output size is bounded by explicit limits.
• Subquery failure does not fail the whole workflow when useful partial data exists.
• Tests cover full success, no bills, and partial report/meeting failure.
• No raw API expansion is required for the first pass.