fix: require full Gmail scope for permanent deletes

[caioribeiroclw-pixel]

Fixes #38.

users.messages.delete / users.messages.batchDelete require the full Gmail mailbox scope (https://mail.google.com/), so exposing permanent-delete tools under gmail.modify creates a bad UX: the tool is visible, then fails at execution time with Insufficient Permission.

This PR keeps the existing default safer:

• adds gmail.full -> https://mail.google.com/
• gates delete_email and batch_delete_emails behind gmail.full
• documents that permanent delete is separate from the default gmail.modify,gmail.settings.basic auth
• recommends modify_email / batch_modify_emails for archive / mark-read / label cleanup instead of permanent delete
• adds regression tests for shorthand + URL scope handling and for hiding permanent delete under gmail.modify

Checks run locally:

• npm test — 115/115 passed
• npm run build
• git diff --check