feat: extend channel_prompts to Telegram, Slack, and Mattermost

Extract resolve_channel_prompt() shared helper into
gateway/platforms/base.py. Refactor Discord to use it.
Wire channel_prompts into Telegram (groups + forum topics),
Slack (channels), and Mattermost (channels).

Config bridging now applies to all platforms (not just Discord).
Added channel_prompts defaults to telegram/slack/mattermost
config sections.

Docs added to all four platform pages with platform-specific
examples (topic inheritance for Telegram, channel IDs for Slack,
etc.).

hermes_cli/config.py

@@ -670,6 +670,21 @@ DEFAULT_CONFIG = {
         # Supports \n for newlines, e.g. "🤖 *My Bot*\n──────\n"
     },
 
+    # Telegram platform settings (gateway mode)
+    "telegram": {
+        "channel_prompts": {},         # Per-chat/topic ephemeral system prompts (topics inherit from parent group)
+    },
+
+    # Slack platform settings (gateway mode)
+    "slack": {
+        "channel_prompts": {},         # Per-channel ephemeral system prompts
+    },
+
+    # Mattermost platform settings (gateway mode)
+    "mattermost": {
+        "channel_prompts": {},         # Per-channel ephemeral system prompts
+    },
+
     # Approval mode for dangerous commands:
     #   manual — always prompt the user (default)
     #   smart  — use auxiliary LLM to auto-approve low-risk commands, prompt for high-risk