Expose repository scanner API

[andrew]

Moves repository scanning into the root Go package so other Go modules can use
the same traversal and report schema as the command. The command now delegates
to the exported API.

Adds opt-in IncludeLegalFiles support for retaining license and notice file
metadata and complete text decoded to UTF-8 without changing command defaults.

Closes #30
Closes #31

[Copilot]

Pull request overview

This pull request exposes the repository scanner through the root Go package, updates the CLI to use it, and adds opt-in legal-file reporting.

Changes:

• Adds exported scanning options, reports, validation, and API.
• Adds IncludeLegalFiles support with metadata.
• Adds API tests and documentation.
• Preserves existing CLI behavior.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scan.go	Exported scanner API and legal-file support
scan_test.go	Migrated scanner tests
scan_api_test.go	External API and legal-file tests
README.md	Documents repository scanning API
corpus.go	Updates package documentation
cmd/licenses/scan.go	Delegates scanning to the root API
cmd/licenses/scan_test_helpers_test.go	Retains CLI test helpers
cmd/licenses/licensee_comparison_test.go	Uses exported legal-file classification

Suppressed comments (1)

scan.go:108

• DefaultScanOptions leaves SkipDirs nil, so a caller customizing the returned defaults with the natural options.SkipDirs["generated"] = true operation will panic. Initialize this map here (as the CLI's parseSkippedDirectories does) so the exported defaults can be safely extended.

	return ScanOptions{
		MaxDepth:    defaultMaxDepth,
		MaxFiles:    defaultMaxFiles,
		MaxFileSize: defaultMaxFileSize,
		Workers:     defaultWorkerCount(),
	}

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