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/denotate.go

@@ -0,0 +1,74 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/spf13/cobra"
+)
+
+var denotateCmd = &cobra.Command{
+	Use:   "denotate [filter...]",
+	Short: "Remove the most recent annotation from a task",
+	Long: `Remove the most recent annotation from a task.
+
+Examples:
+  opal 2 denotate
+  opal denotate +bug`,
+	Run: func(cmd *cobra.Command, args []string) {
+		parsed := getParsedArgs(cmd)
+		if err := denotateTask(parsed.Filters); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+	},
+}
+
+func denotateTask(filterArgs []string) error {
+	if len(filterArgs) == 0 {
+		return fmt.Errorf("no task specified")
+	}
+
+	filter, err := engine.ParseFilter(filterArgs)
+	if err != nil {
+		return fmt.Errorf("failed to parse filter: %w", err)
+	}
+
+	ws, err := engine.LoadWorkingSet()
+	if err != nil {
+		return fmt.Errorf("failed to load working set: %w", err)
+	}
+
+	var task *engine.Task
+
+	if len(filter.IDs) > 0 {
+		if len(filter.IDs) != 1 {
+			return fmt.Errorf("denotate requires exactly one task")
+		}
+		task, err = ws.GetTaskByDisplayID(filter.IDs[0])
+		if err != nil {
+			return err
+		}
+	} else {
+		tasks, err := engine.GetTasks(filter)
+		if err != nil {
+			return fmt.Errorf("failed to get tasks: %w", err)
+		}
+		if len(tasks) == 0 {
+			return fmt.Errorf("no tasks matched filter")
+		}
+		if len(tasks) > 1 {
+			return fmt.Errorf("denotate requires exactly one task (filter matched %d)", len(tasks))
+		}
+		task = tasks[0]
+	}
+
+	removed, err := task.Denotate()
+	if err != nil {
+		return err
+	}
+
+	fmt.Printf("Removed annotation: %s\n", removed.Text)
+	return nil
+}