feat: add task history via log command and info integration

Add engine/history.go with GetTaskHistory and diff-style FormatTaskHistory
that compares consecutive change_log entries to show only what changed.
Add cmd/log.go command for full task history. Integrate last 5 history
entries into FormatTaskDetail (info view) as a "Recent Changes" section.

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

opal-task/cmd/log.go

@@ -0,0 +1,75 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+
+	"git.jnss.me/joakim/opal/internal/engine"
+	"github.com/spf13/cobra"
+)
+
+var logCmd = &cobra.Command{
+	Use:   "log [filter]",
+	Short: "Show change history for a task",
+	Long: `Show the change history for a single task, pulled from the change log.
+
+Examples:
+  opal 2 log
+  opal log +bug`,
+	Run: func(cmd *cobra.Command, args []string) {
+		parsed := getParsedArgs(cmd)
+		if err := showTaskLog(parsed.Filters); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+	},
+}
+
+func showTaskLog(args []string) error {
+	if len(args) == 0 {
+		return fmt.Errorf("no task specified for log command")
+	}
+
+	filter, err := engine.ParseFilter(args)
+	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("log 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("log requires exactly one task (filter matched %d)", len(tasks))
+		}
+		task = tasks[0]
+	}
+
+	entries, err := engine.GetTaskHistory(task.UUID)
+	if err != nil {
+		return err
+	}
+
+	fmt.Printf("History for: %s\n\n", task.Description)
+	fmt.Print(engine.FormatTaskHistory(entries))
+	return nil
+}

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",
+	"version", "annotate", "denotate", "undo", "log",
 }
 
 // Report names (dynamically populated)
@@ -239,6 +239,7 @@ func init() {
 	rootCmd.AddCommand(annotateCmd)
 	rootCmd.AddCommand(denotateCmd)
 	rootCmd.AddCommand(undoCmd)
+	rootCmd.AddCommand(logCmd)
 
 	// Enable --version flag on root command
 	rootCmd.Version = Version