Add JSON metadata for scanner reports

[andrew]

Adds scanner name and binary version to JSON reports. Reported files now include source-byte SHA-256 hashes and legal-file roles, while expressions record whether root legal or README files contributed them.

Schema 2 remains additive. Existing output modes and exit codes are unchanged.

Closes #18
Closes #19
Closes #20
Closes #21

[Copilot]

Pull request overview

Extends schema 2 JSON scanner reports with provenance and file-level metadata while preserving existing output modes and exit codes.

Changes:

• Adds scanner name and binary version.
• Adds SHA-256 hashes and legal-file roles.
• Marks expressions contributed by root legal or README files.

Non-blocking nit: cmd/licenses/scan.go computes legal-file roles before filtering non-reportable files, causing avoidable per-file overhead.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
cmd/licenses/scan.go	Implements report metadata and classification logic.
cmd/licenses/scan_test.go	Tests metadata, hashes, roles, and expression classification.
cmd/licenses/main.go	Passes CLI version into scans.
cmd/licenses/main_test.go	Verifies JSON metadata and output fields.
cmd/licenses/licensee_comparison_test.go	Updates scanner invocation.
cmd/licenses/benchmark_test.go	Updates benchmark invocations.

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