Fix flaky rebind test: targets could collide with generated name

test_plain_claude_rebind_moves_session_binding_to_target rebinds a
plain Claude to "nova" then "vega". Both are gold names the generator
favors, so ~1 in 100 runs start_bare's random identity was already
"nova", recorded with the process cwd, and the rebind was correctly
refused as a directory mismatch. Use names the 4-letter generator can
never produce.