Telegram is the legacy pairing-only mode; combining it with
--discord/--slack/--whatsapp/etc produced confusing startup state where
the dashboard never came up but the extra channels silently ran. Fail
fast via argparse error so the user gets a clear message.
Uses `getattr(args, flag, False)` so unregistered flags don't crash.
Leaves `_check_extras_installed` and the rest of the main() control
flow untouched (earlier iterations of the PR moved it too early and
broke `doctor`/`status`).
Reduces the original test to the conflict cases only — asserting on the
non-conflict path would require mocking the full startup pipeline.
Co-Authored-By: Aravindavenge <119057955+Aravindavenge@users.noreply.github.com>
