feat(mail): add --attachment to send, drafts create and replies

[okuegow]

Problem

mog couldn't attach files to outgoing mail — mail send and mail drafts create only supported recipients, subject and body.

Changes

• New repeatable --attachment <path> flag for mail send and mail drafts create. Each file becomes an inline base64 microsoft.graph.fileAttachment (typed payload struct); content type is inferred from the extension (fallback application/octet-stream).
• Up-front validation with clear errors: empty path, missing file, directory, and files at/over Graph's 3 MB inline limit (larger files would need an upload session).
• Replies (--reply-to-message-id) support attachments too. The single-step /reply action can't carry inline attachments, so the reply is built via createReply (keeping the quoted original), then PATCH (recipients; for HTML also the body), one POST .../attachments per file, and POST .../send. Text replies pass the user's text as the createReply comment; HTML replies prepend the user's HTML before the quoted draft body. On a pre-send failure the orphaned draft is deleted (best effort); if sending fails the draft is kept and named in the error.
• Also fixes the pre-existing non-attachment reply path, which set both comment and message.body (Graph wants one or the other and can return 400): now comment for text, message.body for HTML.

Tests

• buildAttachments: content-type inference, and empty/missing/directory/oversize rejection.
• mail send and mail drafts create POST payloads include the attachment (asserted via the mock client).
• Reply flow end-to-end via the mock client: exact call order createReply → patch → attach → send, attachment/recipient payloads, HTML quote prepending, and the createReply/attach/send error paths incl. orphan-draft cleanup.
• Verified live against a real Microsoft 365 account: attachments on a new mail and a draft, and a reply-with-attachment that arrived with the correct file.
• go build / go vet / go test ./... pass. README and --ai-help updated.

Closes #10