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/undo.go

@@ -0,0 +1,28 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/spf13/cobra"
+)
+
+var undoCmd = &cobra.Command{
+	Use:   "undo",
+	Short: "Undo the last action",
+	Long: `Undo the most recent mutating action (add, done, delete, modify, start, stop).
+
+The undo stack keeps the last 10 operations. Each undo pops one operation.
+
+Examples:
+  opal undo`,
+	Run: func(cmd *cobra.Command, args []string) {
+		description, err := engine.PopUndo()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+		fmt.Println(description)
+	},
+}