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/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