feat: add --attachment flag to mail drafts create and mail send

[okuegow]

Problem

mog mail drafts create and mog mail send currently only accept --to, --subject, --body/--body-file. There is no way to attach a local file (PDF, image, etc.) when composing.

For invoicing/customer workflows where a signed contract PDF should travel with the email, users have to fall back to attaching manually in Outlook after the draft is created — which defeats the point of scripting the draft via mog.

Proposed solution

Add a repeatable --attachment <path> flag to both mail drafts create and mail send. Under the hood this maps to Microsoft Graph's POST /me/messages/{id}/attachments (file attachments under 3 MB) and the upload-session variant for larger files.

mog mail drafts create \
  --to a@example.com \
  --subject "Hi" \
  --body-file mail.txt \
  --attachment ./contract.pdf \
  --attachment ./annex.pdf

Notes

• Graph distinguishes small (< 3 MB inline base64) vs. large (upload session) attachments — both should be handled transparently.
• mail attachment list/download already exist, so the missing piece is an upload command. An mail attachment add <draft-id> <path> subcommand would also work as a lower-level primitive.

[okuegow]

Still relevant for me — and I'd be happy to open a PR if you'd accept it (no rush, I know this is a side project).

Rough plan:

• Add a repeatable --attachment <path> flag to both mail send and mail drafts create.
• For each file, build a microsoft.graph.fileAttachment object (name, contentType inferred from the extension, contentBytes = base64 of the file) and include it in the attachments array of the message payload.
• Keep it simple for now (inline base64, which is fine for the typical small-file case); large-file upload sessions could be a follow-up.

Does that match how you'd want it designed? If so I'll send a PR referencing this issue.