Introduce Agent Service Account type

[domenicsim1]

Prerequisites

• I have searched open and closed issues to make sure it isn't already requested
• My team has started working on this issue
• I have written a descriptive issue title

The enhancement

The Need

Agents operating in Octopus had no recognizable identity. When an AI agent used the product it operated under either a regular user account or a standard service account.

Solution

Add a new Agent Service Account type gives agents a first-class, recognizable identity across the product.

Screenshots and Mockups