Fix bd-373c: Surface daemon errors when multiple .db files exist

When daemon detects multiple .db files (after filtering .backup and vc.db),
it now writes detailed error to .beads/daemon-error file before exiting.

The error file is checked and displayed when:
- Daemon discovery fails to connect
- Auto-start fails to yield a running daemon
- User runs 'bd daemons list'

This makes the error immediately visible without requiring users to check
daemon logs.

Changes:
- cmd/bd/daemon.go: Write daemon-error file on multiple .db detection
- internal/daemon/discovery.go: Read and surface daemon-error in DaemonInfo.Error
- cmd/bd/main.go: Display daemon-error when auto-start fails

Amp-Thread-ID: https://ampcode.com/threads/T-1005a8d1-7a5a-4844-ad2d-2b8a6145825f
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/main.go

@@ -370,6 +370,14 @@ var rootCmd = &cobra.Command{
 						if err != nil {
 							daemonStatus.Detail = err.Error()
 						}
+						// Check for daemon-error file to provide better error message
+						if beadsDir := filepath.Dir(socketPath); beadsDir != "" {
+							errFile := filepath.Join(beadsDir, "daemon-error")
+							if errMsg, readErr := os.ReadFile(errFile); readErr == nil && len(errMsg) > 0 {
+								fmt.Fprintf(os.Stderr, "\n%s\n", string(errMsg))
+								daemonStatus.Detail = string(errMsg)
+							}
+						}
 						if os.Getenv("BD_DEBUG") != "" {
 							fmt.Fprintf(os.Stderr, "Debug: auto-start did not yield a running daemon; falling back to direct mode\n")
 						}