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()

README.md

@@ -85,6 +85,30 @@ uv run timesheets stories -w --joplin -o stories.md  # write to file
 uv run timesheets status --joplin
 ```
 
+## Timesheet table format
+
+Each day's entries live in a markdown pipe table with the following columns:
+
+```markdown
+| Start | End   | Duration | Project | Story     | Note       |
+|-------|-------|----------|---------|-----------|------------|
+| 09:00 | 10:00 | 1:00     | acme    | PROJ-123  | Fix bug    |
+| 10:00 | 10:15 | 0:15     | scrum   |           | dsu        |
+```
+
+### Excluding entries from CSV export
+
+Prefix the project name with `~` to mark an entry as **count-but-don't-export**.
+The entry is included in `summary` and `status` totals, but omitted from `csv` output.
+This is useful for time that is already pre-entered in Odoo (e.g. holidays, days off).
+
+```markdown
+| 09:00 | 17:00 | 8:00 | ~Leave |  | Public holiday |
+```
+
+The `~` is stripped before project name resolution, so `~Leave` maps to the same
+project as `Leave` in your `project_map.json`.
+
 ## Config file
 
 Copy `timesheets.example.toml` to `timesheets.toml` in the working directory