fix(gateway): first-turn intro note must not swallow a real task

The zero-session first-contact sidecar note (PLAIN_INTRO_NOTE /
_hmwa_first_contact_notes) unconditionally told the model to just
introduce itself, with no carve-out for the case where the user's
first-ever message IS a real task. On a fresh tenant whose voice task
was the very first message, the model followed the note verbatim and
replied with a static 'I'm Hermes. /help shows the available commands.'
- no tool call, no attempt at the task at all. This is a third shape of
the first-turn-onboarding-hijack class (turn replaced outright, not
just augmented with the known profile-build pitch).

Fix: PLAIN_INTRO_NOTE now instructs the model to do the task first
(call whatever tools it needs) and fold the one-line intro into the
close of that same reply; only a message with no real request gets the
old bare intro behavior. Also de-duplicated the literal note text in
gateway/run_turn.py, which had drifted into an inline copy instead of
importing agent.onboarding.PLAIN_INTRO_NOTE.

(cherry picked from commit fc7c3839b0b6774133d4fe8df59a9e98d23bdd8b)

gateway/run_turn.py

@@ -1424,11 +1424,8 @@ class GatewayTurnMixin:
         if history:
             return
         if not await self.async_session_store.has_any_sessions():
-            _intro_note = (
-                "[System note: This is the user's very first message ever. "
-                "Briefly introduce yourself and mention that /help shows available commands. "
-                "Keep the introduction concise -- one or two sentences max.]"
-            )
+            from agent.onboarding import PLAIN_INTRO_NOTE
+            _intro_note = PLAIN_INTRO_NOTE
             # onboarding.profile_build == "ask" (default) and not yet offered: swap the plain intro for
             # a consent-gated profile-build directive. Fires at most once.
             try: