Add output filename generation and workbook save flow

src/taetigkeitsnachweis/writer.py

@@ -94,3 +94,56 @@ def build_employee_workbooks(
         workbooks[employee_name] = workbook
 
     return workbooks
+
+
+def build_output_filename(
+    filename_pattern: str,
+    employee_name: str,
+    report_year: int,
+    report_month: int,
+) -> str:
+    first_name, last_name = _split_name_for_filename(employee_name)
+    safe_first = first_name.replace(" ", "_")
+    safe_last = last_name.replace(" ", "_")
+    return filename_pattern.format(
+        year=report_year,
+        month=f"{report_month:02d}",
+        first_name=safe_first,
+        last_name=safe_last,
+    )
+
+
+def save_employee_workbooks(
+    config: dict[str, Any],
+    employee_workbooks: dict[str, Workbook],
+    report_month: int,
+    report_year: int,
+    project_root: Path,
+) -> list[Path]:
+    paths_cfg = config["paths"]
+    output_cfg = config["output"]
+    filename_pattern = output_cfg["filename_pattern"]
+
+    output_dir = project_root / paths_cfg["output_dir"]
+    output_dir.mkdir(parents=True, exist_ok=True)
+
+    saved_paths: list[Path] = []
+    used_names: set[str] = set()
+
+    for employee_name, workbook in employee_workbooks.items():
+        filename = build_output_filename(
+            filename_pattern=filename_pattern,
+            employee_name=employee_name,
+            report_year=report_year,
+            report_month=report_month,
+        )
+        if filename in used_names:
+            raise ValueError(f"Output filename collision detected: {filename}")
+
+        used_names.add(filename)
+        output_path = output_dir / filename
+        workbook.save(output_path)
+        workbook.close()
+        saved_paths.append(output_path)
+
+    return saved_paths