Add --weekly flag to csv command

- Add `write_csv_weekly()` to output.py: writes entries from multiple
  days as a single CSV with one header row, correct date per row
- Add `-w`/`--weekly` flag to csv subparser
- _cmd_csv branches on args.weekly: fetches week sections, formats
  per-day date strings, calls write_csv_weekly; --raw is honoured
- Add TestWriteCsvWeekly with 6 tests
- Update README with weekly csv usage examples

src/timesheets/cli.py

@@ -21,6 +21,7 @@ from .output import (
     print_summary_weekly_totals,
     to_csv_entries,
     write_csv,
+    write_csv_weekly,
 )
 from .parser import (
     aggregate_rows,
@@ -140,6 +141,13 @@ def build_parser() -> argparse.ArgumentParser:
             "combining entries that share the same project and story."
         ),
     )
+    csv_parser.add_argument(
+        "--weekly",
+        "-w",
+        action="store_true",
+        default=False,
+        help="Export the full week containing the given day instead of a single day.",
+    )
 
     status_parser = subparsers.add_parser(
         "status",
@@ -396,18 +404,37 @@ 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)
-    rows = _resolve_rows(args, config, target_date)
-    if not rows:
-        print("Warning: no timesheet rows found in input.", file=sys.stderr)
-    entries = to_csv_entries(rows) if args.raw else aggregate_rows(rows)
     project_map = _resolve_project_map(args, config)
 
-    if args.output:
-        with open(args.output, "w", newline="", encoding="utf-8") as f:
-            write_csv(entries, f, date_str, project_map)
-        print(f"Written to {args.output}", file=sys.stderr)
+    if args.weekly:
+        day_sections_raw = _resolve_week_sections(args, config, target_date)
+        if not day_sections_raw:
+            print("Warning: no timesheet rows found for this week.", file=sys.stderr)
+            return
+        day_sections = [
+            (
+                format_date(day),
+                to_csv_entries(rows) if args.raw else aggregate_rows(rows),
+            )
+            for day, rows in day_sections_raw
+        ]
+        if args.output:
+            with open(args.output, "w", newline="", encoding="utf-8") as f:
+                write_csv_weekly(day_sections, f, project_map)
+            print(f"Written to {args.output}", file=sys.stderr)
+        else:
+            write_csv_weekly(day_sections, sys.stdout, project_map)
     else:
-        write_csv(entries, sys.stdout, date_str, project_map)
+        rows = _resolve_rows(args, config, target_date)
+        if not rows:
+            print("Warning: no timesheet rows found in input.", file=sys.stderr)
+        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)
+            print(f"Written to {args.output}", file=sys.stderr)
+        else:
+            write_csv(entries, sys.stdout, date_str, project_map)
 
 
 # ---------------------------------------------------------------------------

src/timesheets/output.py

@@ -31,6 +31,32 @@ def to_csv_entries(rows: list[dict]) -> list[dict]:
     ]
 
 
+def write_csv_weekly(
+    day_sections: list[tuple[str, list[dict]]],
+    output: IO[str],
+    project_map: dict,
+) -> None:
+    """Write entries from multiple days as a single CSV with one header row.
+
+    day_sections is a list of (date_str, entries) pairs where entries are
+    already write_csv-compatible (project, description, quantity).
+    """
+    writer = csv.writer(output)
+    writer.writerow(["Date*", "Project*", "Task", "Description", "Quantity"])
+    for date_str, entries in day_sections:
+        for entry in entries:
+            project, task = resolve_project_task(entry["project"], project_map)
+            writer.writerow(
+                [
+                    date_str,
+                    project,
+                    task,
+                    entry["description"],
+                    f"{entry['quantity']:.2f}",
+                ]
+            )
+
+
 def write_csv(
     aggregated: list[dict],
     output: IO[str],