feat: add annotations, undo system, and schema updates

Add annotations as JSON column on tasks table with Annotate/Denotate
methods and CLI commands. Add undo system backed by change_log with
lightweight undo_stack table (capped at 10 entries). All mutating CLI
commands (add, done, delete, modify, start, stop) now record undo
entries. Undo restores prior task state from change_log data.

Schema changes (in v1 migration):
- annotations TEXT column on tasks
- undo_stack table
- annotations field in change_log triggers

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",
+	"version", "annotate", "denotate", "undo",
 }
 
 // Report names (dynamically populated)
@@ -45,8 +45,9 @@ var reportNames = []string{
 }
 
 var commandsWithModifiers = map[string]bool{
-	"add":    true,
-	"modify": true,
+	"add":      true,
+	"modify":   true,
+	"annotate": true,
 }
 
 var rootCmd = &cobra.Command{
@@ -235,6 +236,9 @@ func init() {
 	rootCmd.AddCommand(editCmd)
 	rootCmd.AddCommand(reportsCmd)
 	rootCmd.AddCommand(versionCmd)
+	rootCmd.AddCommand(annotateCmd)
+	rootCmd.AddCommand(denotateCmd)
+	rootCmd.AddCommand(undoCmd)
 
 	// Enable --version flag on root command
 	rootCmd.Version = Version