Centralize BD_DEBUG logging into internal/debug package

- Created internal/debug package with Enabled(), Logf(), Printf()
- Added comprehensive unit tests for debug package
- Replaced 50+ scattered os.Getenv("BD_DEBUG") checks across 9 files
- Centralized debug logic for easier maintenance and testing
- All tests passing, behavior unchanged

Closes bd-fb95094c.5

internal/debug/debug.go

@@ -0,0 +1,24 @@
+package debug
+
+import (
+	"fmt"
+	"os"
+)
+
+var enabled = os.Getenv("BD_DEBUG") != ""
+
+func Enabled() bool {
+	return enabled
+}
+
+func Logf(format string, args ...interface{}) {
+	if enabled {
+		fmt.Fprintf(os.Stderr, format, args...)
+	}
+}
+
+func Printf(format string, args ...interface{}) {
+	if enabled {
+		fmt.Printf(format, args...)
+	}
+}