feat: add assistant file attachments

[Microck]

What changed

• add repeated --attach <PATH> flags to kagi assistant
• keep the existing JSON assistant prompt path for normal prompts
• switch to multipart uploads when attachments are present, matching Kagi's live assistant transport
• load local files from disk with inferred MIME types before upload
• add parser, payload, and multipart request tests
• add a README example for attached assistant prompts

Verification

• cargo test -- --nocapture
• live single-file upload returned attached-note
• live multi-file upload returned 2

Closes #85