- Add agent discovery step to cmdSetupInteractive flow
- _promptAgentSelection: discover uwf-* binaries, auto-select if only one,
prompt user to choose if multiple, show install hints if none found
- mergeConfig: always write selected agent entry, update defaultAgent
- Known agent labels for hermes, claude-code, cursor, builtin
- 10 new tests for _agentNameFromBinary, _printAgentMenu, cmdSetup agent config
Fixes#424