get: print short_text body inline with headers

[Copilot]

get only showed headers, requiring a separate get-data call to read even a short plain-text message body.

Changes

• internal/api/client.go — added ShortText *string \json:"short_text"`toMessage`; the field was already returned by the API but silently dropped during JSON decode
• cmd/get.go — after headers, prints a blank line followed by short_text when present; users only need get-data for non-text or oversized messages
• README.md — updated get command description to reflect the new output

Example output

From: @alice@example.com
To:   ["@bob@example.com"]
Type: text/plain
Size: 12
Time: 1746523200.000000

Hello, world!