feat: listMessageStreams tool #29

Context

Survey request #5. There's no tool to enumerate a server's message streams today — users must already know stream IDs. getServerInfo returns server settings but not the stream list.

Proposal

New listMessageStreams tool wrapping the Postmark message-streams list endpoint (SDK getMessageStreams). Return each stream's ID, Name, MessageStreamType (Transactional / Broadcast / Inbound), and description. Optional filter by type.

Acceptance criteria

• Tool registered with a Zod schema (optional streamType filter)
• Formatted output lists ID / name / type per stream
• README entry + read-only smoke-test check

Pairs with per-send stream override (sibling issue). Source: user survey.