Implement opal-task Phase 3: Filter and Modifier Parsing

- Add filter.go: Parse filters (+tag, -tag, attribute:value, IDs)
- Implement Filter.ToSQL() for WHERE clause generation
- Add modifier.go: Parse modifiers (set/clear attributes, add/remove tags)
- Implement Modifier.Apply() to update existing tasks
- Add dateparse.go: Smart date parsing (ISO, today, tomorrow, weekdays)
- Implement nextWeekday logic (smart Sunday interpretation)
- Update GetTasks() to accept Filter parameter
- Add CreateTaskWithModifier() for task creation with modifiers
- Add comprehensive test suite (13 new tests, all passing)
- Support filtering by status, project, priority, tags, UUIDs, display IDs
- Support modifying priority, project, dates, recurrence, tags

opal-task/internal/engine/dateparse.go

@@ -0,0 +1,63 @@
+package engine
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+// ParseDate parses date strings with smart interpretation
+// Supports: ISO dates, relative (tomorrow, today), weekdays (sun, monday)
+func ParseDate(s string) (time.Time, error) {
+	s = strings.ToLower(strings.TrimSpace(s))
+	now := timeNow()
+
+	// Try ISO format first
+	if t, err := time.Parse("2006-01-02", s); err == nil {
+		return t, nil
+	}
+
+	// Relative dates
+	switch s {
+	case "today":
+		return time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, now.Location()), nil
+	case "tomorrow":
+		tomorrow := now.AddDate(0, 0, 1)
+		return time.Date(tomorrow.Year(), tomorrow.Month(), tomorrow.Day(), 0, 0, 0, 0, tomorrow.Location()), nil
+	case "yesterday":
+		yesterday := now.AddDate(0, 0, -1)
+		return time.Date(yesterday.Year(), yesterday.Month(), yesterday.Day(), 0, 0, 0, 0, yesterday.Location()), nil
+	}
+
+	// Weekday names
+	weekdays := map[string]time.Weekday{
+		"sun": time.Sunday, "sunday": time.Sunday,
+		"mon": time.Monday, "monday": time.Monday,
+		"tue": time.Tuesday, "tuesday": time.Tuesday,
+		"wed": time.Wednesday, "wednesday": time.Wednesday,
+		"thu": time.Thursday, "thursday": time.Thursday,
+		"fri": time.Friday, "friday": time.Friday,
+		"sat": time.Saturday, "saturday": time.Saturday,
+	}
+
+	if targetWeekday, ok := weekdays[s]; ok {
+		return nextWeekday(now, targetWeekday), nil
+	}
+
+	return time.Time{}, fmt.Errorf("unable to parse date: %s", s)
+}
+
+// nextWeekday returns the next occurrence of the target weekday
+// Smart logic: if today is Thursday and target is Sunday, returns this Sunday
+// If today is Sunday and target is Sunday, returns next Sunday
+func nextWeekday(from time.Time, target time.Weekday) time.Time {
+	// Calculate days until target
+	daysUntil := int(target - from.Weekday())
+
+	if daysUntil <= 0 {
+		daysUntil += 7 // Next week
+	}
+
+	next := from.AddDate(0, 0, daysUntil)
+	return time.Date(next.Year(), next.Month(), next.Day(), 0, 0, 0, 0, next.Location())
+}