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

tests/test_output.py

@@ -3,7 +3,13 @@ import io
 
 import pytest
 
-from timesheets.output import print_stories, print_summary, to_csv_entries, write_csv
+from timesheets.output import (
+    print_stories,
+    print_summary,
+    to_csv_entries,
+    write_csv,
+    write_csv_weekly,
+)
 
 # ---------------------------------------------------------------------------
 # Shared fixtures
@@ -129,6 +135,69 @@ class TestWriteCsv:
         assert rows[1][3] == "ticket 1"
 
 
+# ---------------------------------------------------------------------------
+# write_csv_weekly
+# ---------------------------------------------------------------------------
+
+
+DAY_SECTIONS = [
+    (
+        "22/03/26",
+        [
+            {"project": "bugs", "description": "ticket 1", "quantity": 1.0},
+            {"project": "scrum", "description": "dsu", "quantity": 0.25},
+        ],
+    ),
+    (
+        "23/03/26",
+        [
+            {"project": "bugs", "description": "ticket 2", "quantity": 0.5},
+        ],
+    ),
+]
+
+
+class TestWriteCsvWeekly:
+    def _run(self, day_sections=None, project_map=None):
+        buf = io.StringIO()
+        write_csv_weekly(
+            DAY_SECTIONS if day_sections is None else day_sections,
+            buf,
+            project_map or {},
+        )
+        buf.seek(0)
+        return list(csv.reader(buf))
+
+    def test_header_written_once(self):
+        rows = self._run()
+        assert rows[0] == ["Date*", "Project*", "Task", "Description", "Quantity"]
+        assert sum(1 for r in rows if r[0] == "Date*") == 1
+
+    def test_row_count(self):
+        rows = self._run()
+        assert len(rows) == 1 + 3  # header + 3 entries across 2 days
+
+    def test_correct_date_per_row(self):
+        rows = self._run()
+        assert rows[1][0] == "22/03/26"
+        assert rows[2][0] == "22/03/26"
+        assert rows[3][0] == "23/03/26"
+
+    def test_empty_sections_produce_header_only(self):
+        rows = self._run(day_sections=[])
+        assert rows == [["Date*", "Project*", "Task", "Description", "Quantity"]]
+
+    def test_project_map_applied(self):
+        rows = self._run(project_map=PROJECT_MAP)
+        assert rows[1][1] == "[Factry] Historian"
+        assert rows[1][2] == "[Historian] Bugs"
+
+    def test_quantity_format(self):
+        rows = self._run()
+        assert rows[1][4] == "1.00"
+        assert rows[3][4] == "0.50"
+
+
 # ---------------------------------------------------------------------------
 # print_summary
 # ---------------------------------------------------------------------------