chore: add missing ASF license headers

[kayemkim]

Summary

The bulk header pass in #242 (July) covered the tree at the time, but 22 source files added since then have no ASF header, and nothing in CI checks for it:

• cli/internal/plugin/*.go (5 files, the plugin discovery and invoke code)
• converters/sigma/: the Python package, its tests and pyproject.toml (15 files)
• converters/nvidia/tests/fixtures/*.yaml (2 files)

This adds the header to each, using the exact text and comment style of the neighbouring files (the Go files share the wrapping used in cli/cmd/*.go, the Python, TOML and YAML files use the # form). Insertion only, no other change; git diff --stat shows 22 files, +368, -0.

The sigma and nvidia suites fail on this branch in the same three and two tests as on current main (OssieDocument and the generated document still expect the semantic_model wrapper removed in #383); #396 migrates them, and with #396 merged on top of this branch both suites pass in full (91 and 83). The cli suite passes.

With the 0.3.0 source release being prepared, I've also started a thread on dev@ about whether a header-only check in CI is wanted, separate from the rest of #258. This PR stands on its own either way.

Written with LLM assistance for the file scan and the insertion script; I checked every file and the header text against its neighbours myself.

Related Issues

Relates to #242, #258, #396.

Checklist

Specification

• Spec changes are included in core-spec/ and follow the existing structure
• Spec changes have been discussed on the mailing list or in a linked issue
• Breaking changes to the spec are clearly called out in the summary

Ontology

• Ontology changes in ontology/ are consistent with spec changes
• New or modified terms are defined and documented

Converters

• Converter logic in converters/ is updated to reflect spec or ontology changes
• New converters include tests under the converter's test directory

Validation

• Validation rules in validation/ are updated if the spec changed
• New validation cases are covered by tests

Documentation

• docs/ is updated to reflect any user-facing changes
• New features or behaviors are documented with examples where appropriate
• CONTRIBUTING.md is updated if the contribution process changed

Examples

• examples/ are added or updated for any new spec constructs or converter support

Tests

• All existing tests pass (pytest / CI green)
• New functionality is covered by tests

Compliance

• ASF license headers are present on all new source files
• No third-party dependencies are added without PMC/IPMC approval