refactor: rewrite framework detection with modular detector architecture

- create detector interface and registry for extensibility
- extract detectors to separate files: backend.go, frontend.go, cms.go, meta.go
- reduce detect.go from 785 lines to 178 lines (pure orchestrator)
- export VersionMatch and ExtractVersionOptimized for detector use
- create result.go with NewFrameworkResult and WithVulnerabilities helpers
- add url validation to New() for early error detection
- add sif_test.go with main package tests
- update detect_test.go to use external test package pattern

cmd/sif/main.go

@@ -16,6 +16,9 @@ import (
 	"github.com/charmbracelet/log"
 	"github.com/dropalldatabases/sif"
 	"github.com/dropalldatabases/sif/pkg/config"
+
+	// Register framework detectors
+	_ "github.com/dropalldatabases/sif/pkg/scan/frameworks/detectors"
 )
 
 func main() {