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/version_tracking.go

@@ -0,0 +1,103 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/steveyegge/beads/internal/beads"
+	"github.com/steveyegge/beads/internal/configfile"
+)
+
+// trackBdVersion checks if bd version has changed since last run and updates metadata.json.
+// This function is best-effort - failures are silent to avoid disrupting commands.
+// Sets global variables versionUpgradeDetected and previousVersion if upgrade detected.
+//
+// bd-loka: Built-in version tracking for upgrade awareness
+func trackBdVersion() {
+	// Find the beads directory
+	beadsDir := beads.FindBeadsDir()
+	if beadsDir == "" {
+		// No .beads directory found - this is fine (e.g., bd init, bd version, etc.)
+		return
+	}
+
+	// Load current config
+	cfg, err := configfile.Load(beadsDir)
+	if err != nil {
+		// Silent failure - config might not exist yet
+		return
+	}
+	if cfg == nil {
+		// No config file yet - create one with current version
+		cfg = configfile.DefaultConfig()
+		cfg.LastBdVersion = Version
+		_ = cfg.Save(beadsDir) // Best effort
+		return
+	}
+
+	// Check if version changed
+	if cfg.LastBdVersion != "" && cfg.LastBdVersion != Version {
+		// Version upgrade detected!
+		versionUpgradeDetected = true
+		previousVersion = cfg.LastBdVersion
+	}
+
+	// Update metadata.json with current version (best effort)
+	// Only write if version actually changed to minimize I/O
+	if cfg.LastBdVersion != Version {
+		cfg.LastBdVersion = Version
+		_ = cfg.Save(beadsDir) // Silent failure is fine
+	}
+}
+
+// getVersionsSince returns all version changes since the given version.
+// If sinceVersion is empty, returns all known versions.
+// Returns changes in chronological order (oldest first).
+func getVersionsSince(sinceVersion string) []VersionChange {
+	if sinceVersion == "" {
+		// Return all versions
+		return versionChanges
+	}
+
+	// Find the index of sinceVersion
+	startIdx := -1
+	for i, vc := range versionChanges {
+		if vc.Version == sinceVersion {
+			startIdx = i
+			break
+		}
+	}
+
+	if startIdx == -1 {
+		// sinceVersion not found in our changelog - return all versions
+		// (user might be upgrading from a very old version)
+		return versionChanges
+	}
+
+	// Return versions after sinceVersion (don't include sinceVersion itself)
+	if startIdx+1 < len(versionChanges) {
+		return versionChanges[startIdx+1:]
+	}
+
+	// No new versions
+	return []VersionChange{}
+}
+
+// maybeShowUpgradeNotification displays a one-time upgrade notification if version changed.
+// This is called by commands like 'bd ready' and 'bd list' to inform users of upgrades.
+// Returns true if notification was shown.
+func maybeShowUpgradeNotification() bool {
+	// Only show if upgrade detected and not yet acknowledged
+	if !versionUpgradeDetected || upgradeAcknowledged {
+		return false
+	}
+
+	// Mark as acknowledged so we only show once per session
+	upgradeAcknowledged = true
+
+	// Display notification
+	fmt.Printf("🔄 bd upgraded from v%s to v%s since last use\n", previousVersion, Version)
+	fmt.Println("💡 Run 'bd upgrade review' to see what changed")
+	fmt.Println()
+
+	return true
+}