Walks a repo and extracts call-graph defs/edges from every file
concurrently via Task.Supervisor.async_stream_nolink, mirroring
Beamscope.Chunking.Pipeline.chunk_repo/2 (same file discovery,
same concurrency model, same :telemetry events under a
[:beamscope, :callgraph_repo, ...] prefix). Kept as a separate
pipeline from chunking rather than folded into it, since defs/edges
accumulate differently (into one shared graph) than chunks (into one
flat list).