feat: reference attachments (OneDrive/SharePoint file links)

[nathanschram]

Summary

Add support for attaching OneDrive/SharePoint files as reference attachments in send-email and draft. Cloud attachments avoid size limits and enable real-time collaboration on shared files.

Motivation

ryaker/outlook-mcp and XenoXilus/outlook-mcp both support OneDrive operations. Cloud-first attachment workflows are increasingly common in enterprise — sharing a link to a file rather than a copy avoids version conflicts and bypasses the 3MB direct attachment limit.

Implementation

Graph API: referenceAttachment resource type

• @odata.type: #microsoft.graph.referenceAttachment
• sourceUrl: OneDrive/SharePoint file URL
• providerType: oneDriveConsumer, oneDriveBusiness, dropbox, other
• permission: view, edit, anonymousView, anonymousEdit, organizationView, organizationEdit

Changes to existing tools:

• send-email: Add cloudAttachments parameter (array of OneDrive/SharePoint URLs)
• draft: Add cloudAttachments parameter

Optional companion: list-drive-files action or tool to browse OneDrive root/recent files for URL discovery. Keep minimal — we're not a file manager.

Permissions: Requires Files.Read or Files.ReadWrite scope for OneDrive access.

Effort

Medium — reference attachment creation + optional OneDrive file listing.

Acceptance Criteria

• send-email and draft accept cloudAttachments parameter
• Creates proper referenceAttachment objects with OneDrive URLs
• Supports permission levels (view, edit)
• Works with both personal OneDrive and OneDrive for Business
• Dry-run shows attachment details
• Unit tests