Validate empty source paths to prevent project root inclusion (#1601)

* Add .context to .gitignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Validate empty source paths to prevent project root inclusion (#1595)

Empty/null source entries (e.g. bare `-` in YAML) resolve to the project
root, causing extreme memory usage. Add validation to reject them with a
clear error message.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>