Add support for daily, weekly, monthly, yearly recurrence patterns

Previously, only numeric forms (1d, 1w, 1m, 1y) and plural word forms
(days, weeks, months, years) were supported for recurrence patterns.
The common adverbial forms (daily, weekly, monthly, yearly) would fail
with 'invalid recurrence pattern' error.

Now users can use natural language recurrence patterns:
- recur:daily   -> 1 day interval
- recur:weekly  -> 7 day interval
- recur:monthly -> 30 day interval
- recur:yearly  -> 365 day interval

Added test coverage for all four new patterns.

opal-task/internal/engine/recurrence_test.go

@@ -18,6 +18,11 @@ func TestParseRecurrencePattern(t *testing.T) {
 		{"2w", 14 * 24 * time.Hour},
 		{"1m", 30 * 24 * time.Hour},
 		{"1y", 365 * 24 * time.Hour},
+		// Test word forms
+		{"daily", 24 * time.Hour},
+		{"weekly", 7 * 24 * time.Hour},
+		{"monthly", 30 * 24 * time.Hour},
+		{"yearly", 365 * 24 * time.Hour},
 	}
 
 	for _, tt := range tests {