Support repo-level analysis with transitive dependency graph

### Description

The tool currently operates at the solution level, which doesn't work for teams with mono-repos or repositories containing multiple
solutions with shared dependencies. Teams need the repo to be the unit of work, not the solution.

The tool should be able to discover all solutions and projects within a repo, build a full transitive dependency graph across them (projects, packages,
cross-solution references), and use that graph to produce a unified repo-level assessment with recommended migration waves and safe upgrade sequencing.

Without this, teams must manually coordinate which solutions to analyze and in what order, and risk upgrading a project before its dependencies are ready
or missing shared libraries that need to be upgraded first.