Support assigning issues to and interacting with the "Project Padawan" SWE agent

[timrogers]

With Project Padawan (external blog link) we're building an autonomous, asynchronous SWE agent integrated into issues and PRs on GitHub.com.

You give Copilot a task by assigning an issue to Copilot via the normal issues UI - and under the hood, this is an assignment to a bot called copilot-swe-agent.

You can then interact with the agent using pull request reviews - including the ability to @mention it as you can a human.

Supporting this well in the extension will include a bunch of different steps:

• Allow assigning an issue to Copilot where it is allowed in the repo
• In the assignee picker, show the name "Copilot", not copilot-swe-agent
• When Copilot is assigned to an issue, render the name "Copilot", not copilot-swe-agent
• When Copilot is shown as the author for a PR, render the name "Copilot", not copilot-swe-agent
• When Copilot is shown as the author of a comment, render the name "Copilot", not copilot-swe-agent
• Allow @mentioning Copilot with the normal autocomplete experience

[timrogers]

I've pushed #6794 which addresses the When Copilot is shown as the author of a comment, render the name "Copilot", not copilot-swe-agent point.

[alexr00]

I have most of these on my list of things to do this week (as long as the GitHub API has a way to do it or I have a way to hack around it). Thanks for jumping in and helping!