fix: reject unknown CLI args unless running in --reflect mode

[Kailigithub]

Problem

agentmain.py used parse_known_args(), which silently accepted unknown
CLI flags. A typo like --goal (intended for a separate Hive launcher)
was swallowed without warning, causing the process to fall through into
the interactive agent loop in the background — the TUI looked frozen
because no master dispatch was ever published.

Reported in #566.

Fix

Add a guard after parse_known_args():

• If unknown args are present and --reflect is not set, call
  parser.error(...) so argparse prints usage and exits non-zero.
• The --reflect path is unchanged: extra --key value pairs are still
  forwarded to the reflect script as _reflect_args.

Verification

Local tests with a synthetic argv (no API keys required):

Test 1 — unknown --goal without --reflect:
  exit code: 2
  stderr:    usage: agentmain.py ...
             agentmain.py: error: unrecognized arguments: --goal /tmp/foo.json

Test 2 — --reflect with extra args (--base_url / --board_key):
  exit code: 0
  OK args= Namespace(... reflect='reflect/agent_team_master.py' ...) unknown= ['--base_url', '...']

Test 3 — no unknown args (--nobg only):
  argparse stage passes; later GeneraticAgent() raises unrelated to this change

ruff check agentmain.py --select F,E9 reports only pre-existing
findings (F541 f-string in another function, F401 side-effect readline
import) — none introduced by this patch.

Closes #566