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

pkg/importer/testdata/trufflehog-sample.json

@@ -0,0 +1,57 @@
+[
+  {
+    "SourceID": 1,
+    "SourceName": "git-scan",
+    "SourceMetadata": {
+      "Data": {
+        "Git": {
+          "commit": "deadbeef1234",
+          "file": "src/config.py",
+          "repository": "https://github.com/example/repo",
+          "line": 42
+        }
+      }
+    },
+    "DetectorName": "OpenAI",
+    "DetectorType": 17,
+    "Verified": true,
+    "Raw": "sk-proj-abcdef1234567890abcdef",
+    "Redacted": "sk-proj-abcd...cdef",
+    "ExtraData": {}
+  },
+  {
+    "SourceID": 2,
+    "SourceName": "fs-scan",
+    "SourceMetadata": {
+      "Data": {
+        "Filesystem": {
+          "file": "/tmp/leaked.env"
+        }
+      }
+    },
+    "DetectorName": "AnthropicV2",
+    "DetectorType": 92,
+    "Verified": false,
+    "Raw": "sk-ant-api03-xxxxxxxxxxxxxxxx",
+    "Redacted": "sk-ant-api03-xxxx",
+    "ExtraData": {}
+  },
+  {
+    "SourceID": 3,
+    "SourceName": "github-scan",
+    "SourceMetadata": {
+      "Data": {
+        "Github": {
+          "link": "https://github.com/foo/bar/blob/main/a.yml",
+          "repository": "https://github.com/foo/bar"
+        }
+      }
+    },
+    "DetectorName": "AWS",
+    "DetectorType": 2,
+    "Verified": true,
+    "Raw": "AKIAIOSFODNN7EXAMPLE",
+    "Redacted": "AKIA****EXAMPLE",
+    "ExtraData": {}
+  }
+]