Expand MCP mail capabilities

[olegberman]

What changed

• expand the MCP server to 18 mail tools across read, write, and send scopes
• add conversations, threads, attachment retrieval, and message/conversation state updates
• add complete draft management and staged attachment transfer
• add rich send/reply support and server-owned forwarding with original attachments
• default newly registered MCP clients to the complete mail scope set
• publish release metadata and notes for HQBase 0.1.26

User impact

New MCP connections receive the complete mail-agent surface after consent. Existing read-only clients remain least-privilege and must reconnect to request the broader scopes.

Workspace administration, infrastructure, audit-history, and Cloudflare credential operations remain outside MCP v1.

Validation

• CI=true pnpm check
  • 225 unit tests
  • 17 Worker integration tests
  • coverage, architecture, TypeScript, Biome, and production build
• CI=true pnpm deploy:dry-run
• git diff --check