Monorepo and multi-repository Tasks

A single Task can now work across more than one connected repository, including monorepos and related services.

Choose the repositories for the Task and Alloyqa tracks each checkout, branch, set of checks, and pull request while keeping the work under one Task.

Changes that span an application and its supporting services can stay coordinated instead of being split across disconnected requests. Each repository still has its own branch and pull-request lifecycle.