Remove the greeting feature entirely

Drops the CONF_GREETING option, the first-contact greeting logic and its
RuntimeData state, and all related strings/translations/README text. Any
greeting value left in an existing entry's options is simply ignored.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -105,8 +105,6 @@ will answer.
   will answer.
 - **Default recipient** – address the `notify.reticulum_send_message` entity
   sends to (see Entities below).
-- **Greeting** – optional message sent automatically the first time each sender
-  contacts Home Assistant (once per sender, reset on restart). Blank = disabled.
 - **Default delivery method** – `direct`, `opportunistic`, or `propagated`.
 - **Announce interval** – periodic re-announce (seconds; `0` disables).
 - **Propagation node** + **sync interval** – optional store-and-forward node so