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>

go.mod

@@ -38,6 +38,7 @@ require (
 	github.com/tidwall/pretty v1.2.1 // indirect
 	github.com/tidwall/sjson v1.2.5 // indirect
 	go.yaml.in/yaml/v3 v3.0.4 // indirect
+	golang.org/x/term v0.37.0 // indirect
 	golang.org/x/text v0.30.0 // indirect
 	golang.org/x/tools v0.38.0 // indirect
 )