Add ~ marker to exclude entries from CSV export

Prefix the project name or note column with ~ to mark an entry as
count-but-don't-export. Marked entries are included in summary and
status totals but omitted from all csv output (both --raw and
aggregated, single-day and weekly).

  | 09:00 | 17:00 | 8:00 | ~Leave |  | Day off  |
  | 09:00 | 17:00 | 8:00 |  Leave |  | ~Day off |

The ~ is stripped from whichever field carries it before any
downstream processing, so project map resolution is unaffected.

Implementation:
- parse_table sets skip_csv=True on marked rows and strips the ~
- new filter_skip_csv() helper in parser.py
- to_csv_entries() skips skip_csv rows
- _cmd_csv calls filter_skip_csv() before aggregate_rows()

tests/test_output.py

@@ -72,6 +72,19 @@ class TestToCsvEntries:
         rows = [_raw_row("  bugs  ", "", "dsu", 0.25)]
         assert to_csv_entries(rows)[0]["project"] == "bugs"
 
+    def test_skips_skip_csv_rows(self):
+        row = {**_raw_row("Leave", "", "Day off", 8.0), "skip_csv": True}
+        assert to_csv_entries([row]) == []
+
+    def test_skip_csv_row_mixed_with_normal(self):
+        rows = [
+            {**_raw_row("Leave", "", "Day off", 8.0), "skip_csv": True},
+            _raw_row("bugs", "ticket 1", "", 1.0),
+        ]
+        entries = to_csv_entries(rows)
+        assert len(entries) == 1
+        assert entries[0]["project"] == "bugs"
+
     def test_mixed_open_and_closed(self):
         rows = [
             _raw_row("bugs", "ticket 1", "", 1.0),