feat: add Bills API tools (search_bills, get_bill_stages)

[MarkDunne]

Summary

• Adds search_bills tool — search bills by title, house, and status; returns current stage with sitting dates and sponsors
• Adds get_bill_stages tool — get the full legislative history of a bill (all readings, committee stages, royal assent) with dates
• Adds request_bills_api utility in utils.py following the existing request_members_api pattern

Motivation

A production user asked "when is the second reading for Preet Gill's Private Member's Bill?" and the model answered from training data (March 2025) instead of the correct date (17 April 2026). No bill-tracking tools existed, so the model had no way to look up live bill status.

Response shaping

Responses are carefully trimmed to balance LLM context:

Kept: billId, title, URL, current house, stage description, sitting dates, sponsor name/party/constituency, isAct/isDefeated status

Stripped: Internal IDs (stageId, sessionId, billStageId), lastUpdate, billTypeId, session IDs, member photos/page URLs, sortOrder, promoters/agent/petitioning fields

Example output

// search_bills(SearchTerm="Public Body Ethnicity Data")
[{
  "billId": 3901,
  "title": "Public Body Ethnicity Data (Inclusion of Jewish and Sikh Categories) Bill",
  "url": "https://bills.parliament.uk/bills/3901",
  "currentHouse": "Commons",
  "currentStage": { "stage": "2nd reading", "abbreviation": "2R", "house": "Commons", "dates": ["2026-04-17"] }
}]

Test plan

• Manual end-to-end test: search_bills and get_bill_stages return correct data for known bills
• Lint passes (ruff check)
• Verify tools are available via MCP client in dev

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