feat: add export-attachment command

[chgeuer]

Adds export-attachment command to export attachment binaries by content hash.

msgvault export-attachment <hash> -o file.pdf   # to file
msgvault export-attachment <hash> -o -          # binary stdout
msgvault export-attachment <hash> --base64      # base64 stdout
msgvault export-attachment <hash> --json        # JSON with base64

This e.g. allows me to easily store all attachments for a given e-mail:

msgvault show-message 45 --json | \
     jq -r '.attachments[] | "\(.content_hash)\t\(.filename)"' | \
     while IFS=$'\t' read -r hash name; do
       msgvault export-attachment "$hash" -o "$name"
     done

Closes #3

[wesm]

Makes sense. If you can bear with me, I'd like to holistically improve the CLI for interacting with file attachments. There is already the get_attachment MCP tool (https://www.msgvault.io/usage/chat/#available-tools), so I think it should be easy to list attachment hashes and export them via the CLI without having to wrangle JSON with jq.

[chgeuer]

Would be awesome to have some 'pure' (non-LLM/MCP) CLI mechanism. I'm calling it from an Elixir application; I didn't want my app to query the real GMail endpoint, but to have a locally cached copy.

[wesm]

I'm back looking at this. Since I just added an export_attachment MCP tool I'll make sure there is some feature parity on this

[wesm]

I don't want to force push your main branch so I created #56 to supersede this