feat: add cobra CLI with input/output flags and debug option

- Add --input/-i and --output/-o flags for file selection
- Add --debug flag to print intermediate JSON to stderr
- Print errors during parsing

go.mod

@@ -1,3 +1,10 @@
 module git.donadeo.net/pdonadeo/todotxt2remind
 
 go 1.24.1
+
+require github.com/spf13/cobra v1.10.1
+
+require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+)