feat(07-01): Importer interface and TruffleHog v3 JSON adapter
- pkg/importer/importer.go: shared Importer interface (Name, Import)
- pkg/importer/trufflehog.go: TruffleHogImporter with v3 JSON decoding,
detector-name normalization (OpenAI/GithubV2/AWS -> canonical ids),
SourceMetadata path+line extraction for Git/Filesystem/Github
- pkg/importer/testdata/trufflehog-sample.json: 3-record fixture
- pkg/importer/trufflehog_test.go: Name, Import, NormalizeName, EmptyArray,
InvalidJSON tests -- all passing
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