fix: IMP-4/5/6 — parser allowlist, delete ID resolution, consistent errors

IMP-5: Replace strings.Contains(arg, ":") heuristic with an allowlist
of recognized attribute keys (ValidAttributeKeys). Colons in task
descriptions (URLs, "Meeting: topic") are no longer misinterpreted as
modifiers. Canonical key sets live in engine/keys.go and are shared
across parseAddArgs, ParseFilter, and ParseModifier. ParseModifier now
errors on unknown keys.

IMP-4: delete command now loads the working set and resolves display IDs
via GetTaskByDisplayID, matching the pattern used by done/modify.

IMP-6: All action commands (done, delete, modify, start, stop) now
return an error on no-match (stderr, exit 1). Previously done/delete
printed to stdout and exited 0; start/stop had no check at all.

Also adds requirements and design docs for the CLI UX improvements.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/cmd/add.go

@@ -71,23 +71,29 @@ func addTask(args []string) error {
 	return nil
 }
 
-// parseAddArgs extracts description and modifiers from args
-// Description = all non-filter/modifier words joined with spaces
-// Filters/Modifiers = args with +, -, or containing :
+// parseAddArgs extracts description and modifiers from args.
+// Tags (+tag, -tag) are always modifiers. For key:value tokens, only
+// recognized attribute keys (engine.ValidAttributeKeys) are treated as
+// modifiers — everything else becomes part of the description.
 func parseAddArgs(args []string) (string, []string, error) {
 	var descParts []string
 	var modifiers []string
 
 	for _, arg := range args {
-		isFilterOrModifier := strings.HasPrefix(arg, "+") ||
-			strings.HasPrefix(arg, "-") ||
-			strings.Contains(arg, ":")
-
-		if isFilterOrModifier {
+		if strings.HasPrefix(arg, "+") || strings.HasPrefix(arg, "-") {
 			modifiers = append(modifiers, arg)
-		} else {
-			descParts = append(descParts, arg)
+			continue
 		}
+
+		if idx := strings.Index(arg, ":"); idx > 0 {
+			key := arg[:idx]
+			if engine.ValidAttributeKeys[key] {
+				modifiers = append(modifiers, arg)
+				continue
+			}
+		}
+
+		descParts = append(descParts, arg)
 	}
 
 	if len(descParts) == 0 {