Introduce slog for structured daemon logging (bd-u2sc.4)

- Replace custom daemonLogger with slog-based structured logging
- Add --log-level flag (debug, info, warn, error) with default "info"
- Add --log-json flag for JSON-formatted log output
- Update log calls to use appropriate levels (Error, Warn, Info, Debug)
- Add test helper functions (newTestLogger, newTestLoggerWithWriter)

Log output examples:
  Text: time=2025-12-23T12:30:00Z level=INFO msg="daemon started" socket=/tmp/bd.sock
  JSON: {"time":"...","level":"INFO","msg":"daemon started","socket":"/tmp/bd.sock"}

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/daemon_integration_test.go

@@ -457,11 +457,7 @@ func TestEventLoopSignalHandling(t *testing.T) {
 
 // createTestLogger creates a daemonLogger for testing
 func createTestLogger(t *testing.T) daemonLogger {
-	return daemonLogger{
-		logFunc: func(format string, args ...interface{}) {
-			t.Logf("[daemon] "+format, args...)
-		},
-	}
+	return newTestLogger()
 }
 
 // TestDaemonIntegration_SocketCleanup verifies socket cleanup after daemon stops