feat: Add built-in version tracking to bd (bd-loka)

Implement automatic bd version tracking and upgrade awareness:

- Add LastBdVersion field to Config struct in metadata.json
- Auto-update version on every bd command in PersistentPreRun
- Add 'bd upgrade' command with status/review/ack subcommands
- Show upgrade notifications on 'bd ready' and 'bd list'
- Non-intrusive: only shows once per session, skipped for JSON output

The system tracks version changes automatically and helps users stay
aware of bd upgrades without manual intervention. Notifications are
graceful - failures don't break commands.

Example output on bd ready after upgrade:
  🔄 bd upgraded from v0.22.0 to v0.24.2 since last use
  💡 Run 'bd upgrade review' to see what changed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

cmd/bd/ready.go

@@ -75,6 +75,10 @@ var readyCmd = &cobra.Command{
 				outputJSON(issues)
 				return
 			}
+
+			// Show upgrade notification if needed (bd-loka)
+			maybeShowUpgradeNotification()
+
 			if len(issues) == 0 {
 				yellow := color.New(color.FgYellow).SprintFunc()
 				fmt.Printf("\n%s No ready work found (all issues have blocking dependencies)\n\n",
@@ -131,6 +135,9 @@ var readyCmd = &cobra.Command{
 			outputJSON(issues)
 			return
 		}
+		// Show upgrade notification if needed (bd-loka)
+		maybeShowUpgradeNotification()
+
 		if len(issues) == 0 {
 			yellow := color.New(color.FgYellow).SprintFunc()
 			fmt.Printf("\n%s No ready work found (all issues have blocking dependencies)\n\n",