WS2: legislation research UI - tool chips, citation pills, LegislationPanel

- shared/types.ts: legislation_tool_call AssistantEvent variant;
  LegislationCitationAnnotation; CitationAnnotation re-widened to a union
  with kind-guards in getDocumentCitationQuotes/expandCitationToEntries/
  formatCitationPage/displayCitationQuote.
- useAssistantChat.ts: legislation_tool_start/result via the MCP
  start/result pairing pattern.
- AssistantMessage.tsx: extract the inline mcp_tool_call chip JSX into a
  shared ToolCallChip (reused, not cloned) and add the legislation_tool_call
  branch (amber "outstanding amendments" note on flagged results);
  toolCallLabel gains the three legislation tool names; citation-source
  helpers (key/label/icon) branch on kind (Landmark icon for legislation).
- New LegislationPanel.tsx: provision heading/text/extent, canonical
  legislation.gov.uk link, prominent amber banner when a lookup reported
  outstanding effects. Degrades honestly (no fabricated "all clear") when
  opened from a citation pill that only carries title/url/quote.
- AssistantSidePanel.tsx: new LegislationTab kind (no documentId, doesn't
  extend CommonTab). ChatView.tsx: openLegislation dedupes tabs by URL
  (bypasses upsertTab's document-keyed dedupe); openCitation branches on
  citation.kind.
- projects/[id]/assistant/chat/[chatId]/page.tsx (a second, simpler
  project-chat surface outside the frontend seam map's scope, no
  LegislationPanel there): falls back to opening the canonical
  legislation.gov.uk link for a legislation citation instead of erroring.

tsc --noEmit clean; eslint 112 problems (35/77), identical to base commit
and zero findings in any file touched here.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>