feat: add attach and download commands, register attachable entity

[drassi]

Summary

We use QBO attachments a lot, especially for Bills to store copies of invoices sent by vendors. I built out the attachable functionality a bit and have been running it for the past week on prod data. Additions:

• Add qbo attach command to upload file attachments to QBO entities (or standalone)
• Add qbo download command to retrieve attachments by ID (saves file or prints pre-signed URL)
• Register attachable as a first-class entity (queryable, creatable, updatable, deletable)
• Update CLI schema, skill docs, and command reference

Details

qbo attach [entity-type entity-id] <file> [--note] [--include-on-send]
Uploads a file via QBO's multipart upload endpoint. Can optionally link it to an entity (e.g., qbo attach invoice 123 receipt.pdf). Supports --dry-run, enforces a 100 MB size limit, and auto-detects content type from the file extension.

qbo download <id> [-o path] [--url]
Downloads an attachment by its Attachable ID. Without -o, uses the original filename from QBO metadata. --url prints the temporary pre-signed download URL instead of saving the file.

Examples

# Upload a receipt and link it to an invoice
qbo attach invoice 123 receipt.pdf --json

# Standalone upload (no entity link)
qbo attach logo.png --json

# Add a note to the attachment
qbo attach bill 456 scan.pdf --note "March utilities"

# Preview without making API calls
qbo attach invoice 123 receipt.pdf --dry-run

# List attachments on an entity
qbo list attachable --where "AttachableRef.EntityRef.value = '123'" --json --results-only

# Download with original filename
qbo download 789

# Save to a custom path
qbo download 789 -o ~/Downloads/receipt.pdf

# Get the pre-signed URL instead of the file
qbo download 789 --url