Split get_attachment into separate attachment + links tools

[imdinu]

Problem

get_attachment changes behavior based on whether filename is provided:

• With filename: extracts and saves an attachment to disk
• Without filename: extracts hyperlinks from the email HTML

This dual behavior is confusing for LLMs — the tool name suggests attachment extraction, but it secretly does link extraction too.

Proposed Solution

Split into two distinct tools:

• get_attachment(message_id, filename) — extract and save attachment
• get_email_links(message_id) — extract hyperlinks from email HTML

Keep get_attachment with filename as optional for backwards compatibility during transition.

Source

Identified via Gemini code review (v0.1.8 development cycle).

[imdinu]

Shipped in v0.2.0.