fix(tools): point email home-channel error at EMAIL_HOME_ADDRESS

The no-home-channel error for send_message derived the env var name
generically as <PLATFORM>_HOME_CHANNEL, producing EMAIL_HOME_CHANNEL for
the email platform. But gateway/config.py reads EMAIL_HOME_ADDRESS, so a
user following the error's guidance would set a variable that is never
consulted. Add a per-platform override map so the email hint names the
variable actually read; all other platforms keep the generic hint.

tools/send_message_tool.py

@@ -44,6 +44,11 @@ _E164_TARGET_RE = re.compile(r"^\s*\+(\d{7,15})\s*$")
 # an explicit target for the email platform, not fall through to channel-name
 # resolution which has no way to resolve a raw address.
 _EMAIL_TARGET_RE = re.compile(r"^\s*[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}\s*$")
+# Most platforms read their home channel from "<PLATFORM>_HOME_CHANNEL", but a
+# few diverge. Email reads EMAIL_HOME_ADDRESS (see gateway/config.py), so the
+# generic "<PLATFORM>_HOME_CHANNEL" hint would point users at a variable that is
+# never read. Map the exceptions so the error guidance is actually actionable.
+_HOME_CHANNEL_ENV_OVERRIDES = {"email": "EMAIL_HOME_ADDRESS"}
 _IMAGE_EXTS = {".jpg", ".jpeg", ".png", ".webp", ".gif"}
 _VIDEO_EXTS = {".mp4", ".mov", ".avi", ".mkv", ".3gp"}
 _AUDIO_EXTS = {".ogg", ".opus", ".mp3", ".wav", ".m4a", ".flac"}
@@ -269,10 +274,13 @@ def _handle_send(args):
             chat_id = home.chat_id
             used_home_channel = True
         else:
+            home_env = _HOME_CHANNEL_ENV_OVERRIDES.get(
+                platform_name, f"{platform_name.upper()}_HOME_CHANNEL"
+            )
             return json.dumps({
                 "error": f"No home channel set for {platform_name} to determine where to send the message. "
                 f"Either specify a channel directly with '{platform_name}:CHANNEL_NAME', "
-                f"or set a home channel via: hermes config set {platform_name.upper()}_HOME_CHANNEL <channel_id>"
+                f"or set a home channel via: hermes config set {home_env} <channel_id>"
             })
 
     duplicate_skip = _maybe_skip_cron_duplicate_send(platform_name, chat_id, thread_id)