Fix GH#367: bd import defaulting to stdin is confusing

Changes:
1. Add TTY detection to bd import - prevents silent hang when run
   interactively without arguments. Shows helpful usage message instead.
2. Fix misleading error messages - change "Run 'bd import'" to
   "Run 'bd sync --import-only'" or explicit file path throughout.

Technical details:
- Added golang.org/x/term dependency for IsTerminal()
- When stdin is a TTY and no -i flag: show usage and exit
- When stdin is piped: works as before (supports script pipelines)
- Preserved all legitimate stdin uses:
  * python gh2jsonl.py --repo owner/repo | bd import
  * python md2jsonl.py feature.md | bd import
  * git show HEAD:.beads/beads.jsonl | bd import -i /dev/stdin

Updated error messages in:
- cmd/bd/staleness.go - main "out of sync" error
- cmd/bd/sync.go - merge completion suggestions
- internal/rpc/server_export_import_auto.go - daemon warnings

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

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

cmd/bd/import.go

@@ -16,6 +16,7 @@ import (
 	"github.com/steveyegge/beads/internal/debug"
 	"github.com/steveyegge/beads/internal/storage/sqlite"
 	"github.com/steveyegge/beads/internal/types"
+	"golang.org/x/term"
 )
 
 var importCmd = &cobra.Command{
@@ -71,6 +72,18 @@ NOTE: Import requires direct database access and does not work with daemon mode.
 		orphanHandling, _ := cmd.Flags().GetString("orphan-handling")
 		force, _ := cmd.Flags().GetBool("force")
 
+		// Check if stdin is being used interactively (not piped)
+		if input == "" && term.IsTerminal(int(os.Stdin.Fd())) {
+			fmt.Fprintf(os.Stderr, "Error: No input specified.\n\n")
+			fmt.Fprintf(os.Stderr, "Usage:\n")
+			fmt.Fprintf(os.Stderr, "  bd import -i .beads/beads.jsonl          # Import from file\n")
+			fmt.Fprintf(os.Stderr, "  bd import -i .beads/beads.jsonl --dry-run # Preview changes\n")
+			fmt.Fprintf(os.Stderr, "  cat data.jsonl | bd import               # Import from pipe\n")
+			fmt.Fprintf(os.Stderr, "  bd sync --import-only                    # Import latest JSONL\n\n")
+			fmt.Fprintf(os.Stderr, "For more information, run: bd import --help\n")
+			os.Exit(1)
+		}
+
 		// Open input
 		in := os.Stdin
 		if input != "" {