feat(gmail): Settings API — vacation, filters, forwarding, IMAP/POP, send-as

[omriariav]

Context

Gmail full API parity (#90) was completed in v1.22.0 but deliberately excluded Settings commands (15+ methods) to keep scope manageable. This issue tracks the remaining Settings API coverage.

Scope

Vacation / Auto-reply (2)

• get-vacation — Get vacation responder settings
• set-vacation — Enable/disable vacation responder (--enabled, --subject, --body, --start, --end, --contacts-only)

Filters (4)

• filters — List mail filters
• filter — Get filter by ID
• create-filter — Create filter (--from, --to, --subject, --has-attachment, --add-labels, --remove-labels, --archive, --trash, --star, --forward)
• delete-filter — Delete a filter

Forwarding (3)

• forwarding-addresses — List forwarding addresses
• create-forwarding — Add forwarding address (--email)
• delete-forwarding — Remove forwarding address (--email)

Send-As / Aliases (4)

• send-as — List send-as aliases
• get-send-as — Get send-as alias details (--email)
• create-send-as — Create send-as alias (--email, --name, --reply-to)
• delete-send-as — Delete send-as alias (--email)

IMAP/POP (2)

• imap-settings — Get IMAP settings
• pop-settings — Get POP settings

Total: ~15 new commands

Requires scopes: gmail.settings.basic, gmail.settings.sharing (already granted in v1.22.0).