[UX] Normalize bill_timeline workflow

[StatPan]

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

Goal

Turn the current bill-history behavior into a stable timeline workflow contract.

This can wrap or replace get_bill_history, but the output should use stable event types and source references.

Scope

• Define event types: introduced, referred_to_committee, committee_meeting, plenary_vote, final_disposition, unknown.
• Include source tool, source ID, date, title, and confidence for each event.
• Preserve compatibility with current bill lookup behavior.
• Use marker payloads when bill details or meetings cannot be found.

Acceptance Criteria

• Timeline event schema is documented in code or tests.
• Existing bill history behavior remains available or is clearly wrapped.
• Tests cover bills with details only, meetings only, and no official match.
• Output sorts events chronologically with stable handling for unknown dates.