feat: JSON, Excel export and time summary

- #23: add JSONFormatter and XLSXFormatter
- add openpyxl dependency for .xlsx export
- #22: add --summary flag and calculate_summary() in report_builder
- ReportRow now carries raw hours for summary calculations
- update CLI help and README with .json/.xlsx formats and --summary
- add tests for new formatters and summary computation

Closes #22, closes #23

pyproject.toml

@@ -23,6 +23,7 @@ dependencies = [
     "tabulate>=0.9.0",
     "python-dotenv>=1.0.0",
     "odfpy>=1.4.0",
+    "openpyxl>=3.1.0",
 ]
 
 [project.optional-dependencies]