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>

internal/rpc/server_export_import_auto.go

@@ -219,7 +219,7 @@ func (s *Server) checkAndAutoImportIfStale(req *Request) error {
 		s.importInProgress.Store(false)
 		shouldDeferRelease = false
 
-		fmt.Fprintf(os.Stderr, "Warning: auto-import skipped - .beads files have uncommitted changes. Run 'bd import' manually after committing.\n")
+		fmt.Fprintf(os.Stderr, "Warning: auto-import skipped - .beads files have uncommitted changes. Run 'bd sync' after committing.\n")
 		return nil
 	}
 
@@ -292,7 +292,7 @@ func (s *Server) checkAndAutoImportIfStale(req *Request) error {
 	err = autoimport.AutoImportIfNewer(importCtx, store, dbPath, notify, importFunc, onChanged)
 	if err != nil {
 		if importCtx.Err() == context.DeadlineExceeded {
-			fmt.Fprintf(os.Stderr, "Error: auto-import timed out after 5s. Run 'bd import' manually.\n")
+			fmt.Fprintf(os.Stderr, "Error: auto-import timed out after 5s. Run 'bd sync --import-only' manually.\n")
 			return fmt.Errorf("auto-import timed out")
 		}
 		// Log but don't fail the request - let it proceed with stale data