feat(wanda): Add dependency resolution for wanda specs

Add buildDepGraph() to parse wanda specs and resolve dependencies by scanning the repo for *.wanda.yaml files. The dependency graph is built in deterministic topological ordering.

Key features:
- Automatic discovery by scanning repo from git root
- Parallel spec parsing for performance
- Cycle detection with helpful error messages
- Variable expansion with unexpanded var detection

Topic: wanda-deps
Relative: wanda-refactor-hello

Signed-off-by: andrew <andrew@anyscale.com>

Author: andrew-anyscale