feat(PROJ-15): CLI Import & Export als Subcommands

- archivmail import: EML + MBOX, --file/--dir/--recursive/--dry-run/--json
- archivmail export: EML + MBOX, Filter --from/--to/--date-from/--date-to/--query/--force/--json
- archivmail help / version
- MBOX Parser (SplitMbox) in pkg/mailparser/mbox.go
- Subcommand-Router in main.go ohne externe Abhängigkeit

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/mailparser/mbox.go

@@ -0,0 +1,43 @@
+package mailparser
+
+import (
+	"bufio"
+	"bytes"
+	"strings"
+)
+
+// SplitMbox splits a raw mbox file into individual RFC 2822 message bytes.
+// Each message starts with a "From " separator line which is skipped.
+func SplitMbox(data []byte) [][]byte {
+	var messages [][]byte
+	var current bytes.Buffer
+
+	scanner := bufio.NewScanner(bytes.NewReader(data))
+	scanner.Buffer(make([]byte, 1024*1024), 10*1024*1024)
+
+	inMessage := false
+	for scanner.Scan() {
+		line := scanner.Text()
+		// mbox separator: line starts with "From " but not "From:" header
+		if strings.HasPrefix(line, "From ") && !strings.HasPrefix(line, "From: ") {
+			if inMessage && current.Len() > 0 {
+				messages = append(messages, bytes.TrimSpace(current.Bytes()))
+				current.Reset()
+			}
+			inMessage = true
+			continue
+		}
+		if inMessage {
+			// unescape ">From " lines (mbox quoting)
+			if strings.HasPrefix(line, ">From ") {
+				line = line[1:]
+			}
+			current.WriteString(line)
+			current.WriteByte('\n')
+		}
+	}
+	if inMessage && current.Len() > 0 {
+		messages = append(messages, bytes.TrimSpace(current.Bytes()))
+	}
+	return messages
+}