docs: add timesheet management documentation

Update README.md with:
- Repository configuration section for .streamd.toml
- Timesheet periods configuration example
- Description of timesheet report features

Update REQUIREMENTS.md with:
- R18a: Timesheet report configuration format
- R18b: Day type rules for expected/actual hours calculation
- R18c: Timesheet report warning types
- Updated R20 command description

README.md

@@ -27,7 +27,34 @@ Within files, `@`-prefixed markers at the beginning of paragraphs or headings de
 
 ## Configuration
 
-Streamd reads its configuration from `~/.config/streamd/config.toml` (XDG standard). The main setting is `base_folder`, which points to the directory containing your stream files (defaults to the current working directory).
+### User Configuration
+
+Streamd reads its user configuration from `~/.config/streamd/config.toml` (XDG standard). The main setting is `base_folder`, which points to the directory containing your stream files (defaults to the current working directory).
+
+### Repository Configuration
+
+For timesheet reporting, create a `.streamd.toml` file in your stream files directory:
+
+```toml
+timezone = "Europe/Berlin"  # Optional: timezone for day boundaries
+
+[timesheet]
+[[timesheet.periods]]
+start = "2026-01-01"
+end = "2026-06-30"
+hours_per_week = 38.0
+
+[[timesheet.periods]]
+start = "2026-07-01"
+end = "2026-12-31"
+hours_per_week = 40.0
+```
+
+The timesheet command will calculate expected vs actual working hours based on these periods, showing:
+- Daily breakdown with expected/actual hours
+- Special day types (sick leave, vacation, holidays, flex days)
+- Warnings for missing entries and overlapping timecards
+- Monthly and cumulative balance
 
 ## Usage