feat: add uncomplete command to restore completed tasks to pending

Dedicated command that sets status back to pending and clears End time.
Unlike undo, works on any completed task regardless of when it was
completed.

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

opal-task/cmd/root.go

@@ -34,7 +34,7 @@ var commandNames = []string{
 	"add", "done", "modify", "delete",
 	"start", "stop", "count", "projects", "tags",
 	"info", "edit", "server", "sync", "reports", "setup",
-	"version", "annotate", "denotate", "undo", "log", "completion",
+	"version", "annotate", "denotate", "undo", "uncomplete", "log", "completion",
 }
 
 // Report names (dynamically populated)
@@ -241,6 +241,7 @@ func init() {
 	annotateCmd.GroupID = "task"
 	denotateCmd.GroupID = "task"
 	undoCmd.GroupID = "task"
+	uncompleteCmd.GroupID = "task"
 	logCmd.GroupID = "task"
 
 	rootCmd.AddCommand(addCmd)
@@ -254,6 +255,7 @@ func init() {
 	rootCmd.AddCommand(annotateCmd)
 	rootCmd.AddCommand(denotateCmd)
 	rootCmd.AddCommand(undoCmd)
+	rootCmd.AddCommand(uncompleteCmd)
 	rootCmd.AddCommand(logCmd)
 
 	// Other commands