feat: initial parser for todo.txt format with spec and test cases

- Add parser for todo.txt tasks supporting priority, dates, projects,
  contexts, and key:value metadata
- Include full todo.txt format specification (SPECS.md) and visual
  description (description.svg)
- Add sample test.todo.txt file with valid, borderline, and malformed
  cases
- Initialize Go module and main entrypoint for parsing and JSON output
- Add .gitignore for binary artifacts

main.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+
+	"git.donadeo.net/pdonadeo/todotxt2remind/internal/parser"
+)
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintf(os.Stderr, "usage: example /path/to/todo.txt\n")
+		os.Exit(2)
+	}
+	f, err := os.Open(os.Args[1])
+	if err != nil {
+		panic(err)
+	}
+	defer func() { _ = f.Close() }()
+
+	tasks, errors := parser.ParseReader(f)
+	if len(errors) > 0 {
+		fmt.Fprintf(os.Stderr, "errors while parsing:\n")
+		for _, e := range errors {
+			fmt.Fprintf(os.Stderr, " - %v\n", e)
+		}
+	}
+	b, _ := json.MarshalIndent(tasks, "", "  ")
+	fmt.Println(string(b))
+}