Refactor solvemail to async nbdev architecture with fastgws OAuth

[ncoop57]

Summary

Rewrites solvemail as a modern async nbdev project using fastgws for OAuth and asyncio for all API calls.

Changes

• Core rewrite: All Gmail API calls are now async/await using fastgws.GMail client, replacing the synchronous google-api-python-client
• Simplified API: Email, Thread, Draft, Label now subclass AttrDict with lazy fetching via fmt argument
• nbdev integration: Project converted to nbdev with _modidx.py, _quarto.yml, nbdev.yml, and autogenerated README/docs
• pyskills support: New skill.py registers solvemail as an LLM skill via pyskills, with read-only methods allowed by default
• Simplified deps: Replaced Google auth stack with fastgws; removed fastship, CHANGELOG.md, legacy Jekyll site files
• New features: EmailAttachment class with lazy fetch(), unsubscribe() via List-Unsubscribe header, forward() on emails/threads, reply_all support

Missing work

The biggest gap are batch operations, but wanted to open the PR early.