fix: reject code identifiers as credentials - #40

Summary

• exclude lowercase calls, dotted references, and symbolic values from generic credential findings
• preserve digit-bearing, provider-shaped, connection-string, and private-key detections
• add focused regressions for Go, Python, and schema-label false positives

Test plan

• swift test --filter DetectionRulesTests
• swift test (1,067 tests)
• swift build -c release
• swiftlint lint --strict
• git diff --check
• dogfood the release binary against false-positive and literal fixtures

Work order: pastewatch/WO-390