sources: remove deprecated pipeline-level sources: (v1.0)

[mostafa]

Pipeline-level sources: blocks were visible-deprecated in #135 and will be hidden from docs in #136. This issue tracks the final removal at v1.0.

Scope

• Hard parse error when a pipeline declares sources:. Error message links to rsigma rule migrate-sources.
• Remove Pipeline.sources field.
• validate_source_refs no longer accepts a pipeline-local declaration scope.
• Dynamic-pipelines test suite reworked to use external source files exclusively.
• Reference page rewritten end-to-end around the external-only model.

Mirrors the CLI command groups removal cycle (#126).

Deprecation timeline

• Shipped in feat: detach dynamic source declarations from pipeline files #135: visible-deprecated with tracing::warn!
• Next release (sources: hide deprecated pipeline-level sources: from docs and examples #136): hidden from docs, louder warning
• v1.0 (this issue): hard parse error, removed entirely