Add the government_agency party role

Regulators and registries that don't tax the company (a division of
corporations, a licensing board), distinct from tax_agency. Core Role,
MCP descriptions, viewer label/noun/icon, docs, and tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

docs/parties-and-attachments.md

@@ -25,7 +25,8 @@ Party {
   name: String,                // legal or trade name, as on documents
   display_name: Option<String>,
   roles: BTreeSet<Role>,       // set by hand: contractor, employee, member, payout_platform,
-                               // tax_agency, financial_institution, landlord (vendor and
+                               // tax_agency, government_agency, financial_institution,
+                               // landlord (vendor and
                                // customer follow the records; see all_roles)
   website: Option<String>,
   address: Option<String>,     // business address; no personal PII beyond it