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()

src/timesheets/output.py

@@ -27,7 +27,7 @@ def to_csv_entries(rows: list[dict]) -> list[dict]:
             "quantity": row["duration_hours"],
         }
         for row in rows
-        if row["duration_hours"] is not None
+        if row["duration_hours"] is not None and not row.get("skip_csv")
     ]