feat: Add get_top_senders analytics tool

[patrickfreyer]

Summary

Add a tool to show who sends the user the most email.

Motivation

Helps identify:

• Key contacts to prioritize
• High-volume senders to filter/unsubscribe
• Email patterns for better organization

Proposed API

@mcp.tool()
def get_top_senders(
    account: str,
    days_back: int = 30,
    mailbox: str = "INBOX",
    limit: int = 10,
    group_by_domain: bool = False
) -> str:
    """Get top email senders by volume.
    
    Returns:
        Ranked list of senders with email counts
        Optionally grouped by domain (e.g., all @company.com)
    """

Use Cases

• "Who emails me most?"
• "Which domains should I create filters for?"
• "Am I getting too many emails from one source?"

Priority

LOW - Nice analytics but not core functionality

Labels

enhancement, priority-low, analytics