Orbit-only — no GitHub code host support #45

Blast Radius depends entirely on GitLab Orbit (orbit sql) for code graph data. There's no alternative backend for:

• GitHub repositories (no equivalent gh dependency-graph)
• Local-only codebases without GitLab
• Bitbucket or other code hosts

The InMemoryOrbitClient exists for testing but requires pre-built graph fixtures — it can't generate a graph from source.

Recommendation: Add a TreeSitterClient that:

1. Walks the local file tree
2. Uses tree-sitter to extract symbol definitions and references
3. Builds an in-memory graph matching the Orbit contract
   This would make Blast Radius work on any codebase without GitLab.