Fix three critical UX issues in opal-task

Issue 1: Fix recurrence calculation for overdue tasks
- Use completion date (End) as base for next instance, not original due date
- If task due Monday completed Wednesday, next is Wednesday+7d not Monday+7d
- Fallback to Due date if End is not set
- Update test to verify new behavior

Issue 2: Fix description parsing to work without quotes
- Add parseAddArgs() to extract description from non-modifier words
- Description = all words that don't start with +, -, or contain :
- Enables: opal add buy groceries +shop carrots → 'buy groceries carrots'
- Validate description is required (error if only modifiers)
- Validate recurring tasks require due date

Issue 3: Implement flexible command syntax
- Add preprocessArgs() to parse arguments before Cobra routing
- Detect command position and split filters (left) from modifiers (right)
- Rewrite os.Args so Cobra routes correctly
- Enable both 'opal 2 done' and 'opal done 2' syntax
- Commands without modifiers accept filters on either side
- Commands with modifiers enforce [filters] command [modifiers]
- Add confirmation for modify without filters (modifies all tasks)

All commands updated to use preprocessed ParsedArgs from context.
All tests passing (33 tests).

opal-task/internal/engine/recurrence_test.go

@@ -209,14 +209,18 @@ func TestSpawnNextInstance(t *testing.T) {
 			found = true
 
 			// Verify due date was advanced
+			// New behavior: calculates from End date (completion time), not original due date
 			if task.Due == nil {
 				t.Error("New instance should have due date")
 			} else {
-				expectedDue := CalculateNextDue(*instance1.Due, duration)
-				// Allow 1 second tolerance due to Unix timestamp precision
-				diff := task.Due.Sub(expectedDue)
-				if diff < -time.Second || diff > time.Second {
-					t.Errorf("Expected due date %v, got %v (diff: %v)", expectedDue, *task.Due, diff)
+				// Should be 7 days from when we completed instance1
+				// Allow reasonable tolerance for test timing
+				diff := task.Due.Sub(time.Now())
+				expectedDiff := duration
+				tolerance := 5 * time.Second
+
+				if diff < expectedDiff-tolerance || diff > expectedDiff+tolerance {
+					t.Errorf("Expected due date ~%v from now, got %v from now (diff: %v)", duration, diff, diff-expectedDiff)
 				}
 			}