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/internal/engine/display.go

@@ -2,6 +2,7 @@ package engine
 
 import (
 	"fmt"
+	"strings"
 	"time"
 
 	"github.com/fatih/color"
@@ -187,6 +188,25 @@ func FormatTaskDetail(task *Task) string {
 		}
 	}
 
+	// Recent changes from change_log (last 5)
+	if entries, err := GetTaskHistory(task.UUID); err == nil && len(entries) > 0 {
+		t.AppendSeparator()
+		// Show last 5 entries
+		start := 0
+		if len(entries) > 5 {
+			start = len(entries) - 5
+		}
+		historyStr := FormatTaskHistory(entries[start:])
+		lines := strings.Split(strings.TrimSpace(historyStr), "\n")
+		for i, line := range lines {
+			label := ""
+			if i == 0 {
+				label = "History"
+			}
+			t.AppendRow(table.Row{label, line})
+		}
+	}
+
 	return t.Render()
 }