Support assigning, referencing Padawan

[chrisdias]

Support @ referencing Copilot in comments and need to support being able to assign a PR to an agent

[chrisdias]

Related: https://github.com/github/sweagentd/issues/450

[alexr00]

This is also not supported yet by GitHub, will add once they have it https://github.com/github/sweagentd/issues/850