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/main.go

@@ -83,6 +83,11 @@ var (
 
 	// Auto-import state
 	autoImportEnabled = true // Can be disabled with --no-auto-import
+
+	// Version upgrade tracking (bd-loka)
+	versionUpgradeDetected = false // Set to true if bd version changed since last run
+	previousVersion        = ""    // The last bd version user had (empty = first run or unknown)
+	upgradeAcknowledged    = false // Set to true after showing upgrade notification once per session
 )
 
 var (
@@ -293,6 +298,10 @@ var rootCmd = &cobra.Command{
 			}
 		}
 
+		// Track bd version changes (bd-loka)
+		// Best-effort tracking - failures are silent
+		trackBdVersion()
+
 		// Initialize daemon status
 		socketPath := getSocketPath()
 		daemonStatus = DaemonStatus{