docs: fix signal-cli install instructions

signal-cli is not available via apt or snap. Replace the incorrect
'sudo apt install signal-cli' with the official install method:
downloading from GitHub releases (Linux) or brew (macOS).

Updated both signal.md docs and the gateway.py setup hint.

Inspired by PR #4225 (which proposed snap, also incorrect).

hermes_cli/gateway.py

@@ -1803,8 +1803,7 @@ def _setup_signal():
         print_warning("signal-cli not found on PATH.")
         print_info("  Signal requires signal-cli running as an HTTP daemon.")
         print_info("  Install options:")
-        print_info("    Linux:  sudo apt install signal-cli")
-        print_info("            or download from https://github.com/AsamK/signal-cli")
+        print_info("    Linux:  download from https://github.com/AsamK/signal-cli/releases")
         print_info("    macOS:  brew install signal-cli")
         print_info("    Docker: bbernhard/signal-cli-rest-api")
         print()

website/docs/user-guide/messaging/signal.md

@@ -25,17 +25,21 @@ The Signal adapter uses `httpx` (already a core Hermes dependency) for all commu
 ### Installing signal-cli
 
 ```bash
-# Linux (Debian/Ubuntu)
-sudo apt install signal-cli
-
 # macOS
 brew install signal-cli
 
-# Manual install (any platform)
-# Download from https://github.com/AsamK/signal-cli/releases
-# Extract and add to PATH
+# Linux (download latest release)
+VERSION=$(curl -Ls -o /dev/null -w %{url_effective} \
+  https://github.com/AsamK/signal-cli/releases/latest | sed 's/^.*\/v//')
+curl -L -O "https://github.com/AsamK/signal-cli/releases/download/v${VERSION}/signal-cli-${VERSION}.tar.gz"
+sudo tar xf "signal-cli-${VERSION}.tar.gz" -C /opt
+sudo ln -sf "/opt/signal-cli-${VERSION}/bin/signal-cli" /usr/local/bin/
 ```
 
+:::caution
+signal-cli is **not** in apt or snap repositories. The Linux install above downloads directly from [GitHub releases](https://github.com/AsamK/signal-cli/releases).
+:::
+
 ---
 
 ## Step 1: Link Your Signal Account