Add --raw flag to csv command to skip aggregation

- Add `to_csv_entries()` to output.py: converts raw rows to write_csv
  entries one-for-one, without merging by (project, description)
- Add `--raw` flag to the csv subparser; _cmd_csv branches on it
- Add TestToCsvEntries with 6 tests
- Update README with --raw usage example
- Add .coverage and htmlcov/ to .gitignore

README.md

@@ -66,6 +66,7 @@ uv run timesheets summary -w -ss --joplin        # weekly totals only: one line
 ```sh
 uv run timesheets csv --joplin                   # stdout
 uv run timesheets csv --joplin -o output.csv     # write to file
+uv run timesheets csv --raw --joplin             # one row per entry, no aggregation
 ```
 
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