Add date_format config key for csv command

- Add get_date_format() to config.py: reads [csv] date_format, returns
  None when absent (falls back to the default %d/%m/%y)
- _cmd_csv applies the format to both single-day and weekly date strings
- Add 3 tests for get_date_format in TestGetters
- Update timesheets.example.toml and README with [csv] date_format key

src/timesheets/cli.py

@@ -6,6 +6,7 @@ from datetime import date
 from .config import (
     find_default_config,
     get_daily_target,
+    get_date_format,
     get_map_path,
     get_token,
     get_weekly_target,
@@ -422,6 +423,10 @@ def _cmd_stories(args: argparse.Namespace, config: dict) -> None:
 def _cmd_csv(args: argparse.Namespace, config: dict) -> None:
     target_date, date_str = _resolve_date(args)
     project_map = _resolve_project_map(args, config)
+    date_fmt = get_date_format(config)
+
+    def _fmt(d: date) -> str:
+        return d.strftime(date_fmt) if date_fmt else format_date(d)
 
     if args.weekly:
         day_sections_raw = _resolve_week_sections(args, config, target_date)
@@ -430,7 +435,7 @@ def _cmd_csv(args: argparse.Namespace, config: dict) -> None:
             return
         day_sections = [
             (
-                format_date(day),
+                _fmt(day),
                 to_csv_entries(rows) if args.raw else aggregate_rows(rows),
             )
             for day, rows in day_sections_raw
@@ -448,10 +453,10 @@ def _cmd_csv(args: argparse.Namespace, config: dict) -> None:
         entries = to_csv_entries(rows) if args.raw else aggregate_rows(rows)
         if args.output:
             with open(args.output, "w", newline="", encoding="utf-8") as f:
-                write_csv(entries, f, date_str, project_map)
+                write_csv(entries, f, _fmt(target_date), project_map)
             print(f"Written to {args.output}", file=sys.stderr)
         else:
-            write_csv(entries, sys.stdout, date_str, project_map)
+            write_csv(entries, sys.stdout, _fmt(target_date), project_map)
 
 
 # ---------------------------------------------------------------------------

src/timesheets/config.py

@@ -54,6 +54,11 @@ def get_map_path(config: dict) -> str | None:
     return config.get("projects", {}).get("map")
 
 
+def get_date_format(config: dict) -> str | None:
+    """Extract csv.date_format from config, or None to use the default."""
+    return config.get("csv", {}).get("date_format")
+
+
 def get_daily_target(config: dict) -> float:
     """Extract work.daily_hours from config, defaulting to 8.0."""
     return float(config.get("work", {}).get("daily_hours", 8.0))