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/staleness.go

@@ -44,14 +44,14 @@ func ensureDatabaseFresh(ctx context.Context) error {
 
 	// Database is stale - refuse to operate
 	return fmt.Errorf(
-		"Database out of sync with JSONL. Run 'bd import' first.\n\n"+
+		"Database out of sync with JSONL. Run 'bd sync --import-only' to fix.\n\n"+
 			"The JSONL file has been updated (e.g., after 'git pull') but the database\n"+
 			"hasn't been imported yet. This would cause you to see stale/incomplete data.\n\n"+
 			"To fix:\n"+
-			"  bd import -i .beads/beads.jsonl  # Import JSONL updates to database\n\n"+
+			"  bd sync --import-only            # Import JSONL updates to database\n"+
+			"  bd import -i .beads/beads.jsonl  # Alternative: specify file explicitly\n\n"+
 			"If in a sandboxed environment (e.g., Codex) where daemon can't be stopped:\n"+
 			"  bd --sandbox ready               # Use direct mode (no daemon)\n"+
-			"  bd import --force                # Force metadata update\n"+
 			"  bd ready --allow-stale           # Skip staleness check (use with caution)\n\n"+
 			"Or use daemon mode (auto-imports on every operation):\n"+
 			"  bd daemon start\n"+