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.).

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

@@ -281,6 +281,23 @@ If this returns your bot's user info, the token is valid. If it returns an error
 
 **Fix**: Add your User ID to `MATTERMOST_ALLOWED_USERS` in `~/.hermes/.env` and restart the gateway. Remember: the User ID is a 26-character alphanumeric string, not your `@username`.
 
+## Per-Channel Prompts
+
+Assign ephemeral system prompts to specific Mattermost channels. The prompt is injected at runtime on every turn — never persisted to transcript history — so changes take effect immediately.
+
+```yaml
+mattermost:
+  channel_prompts:
+    "channel_id_abc123": |
+      You are a research assistant. Focus on academic sources,
+      citations, and concise synthesis.
+    "channel_id_def456": |
+      Code review mode. Be precise about edge cases and
+      performance implications.
+```
+
+Keys are Mattermost channel IDs (find them in the channel URL or via the API). All messages in the matching channel get the prompt injected as an ephemeral system instruction.
+
 ## Security
 
 :::warning

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

@@ -418,6 +418,23 @@ Hermes supports voice on Slack:
 
 ---
 
+## Per-Channel Prompts
+
+Assign ephemeral system prompts to specific Slack channels. The prompt is injected at runtime on every turn — never persisted to transcript history — so changes take effect immediately.
+
+```yaml
+slack:
+  channel_prompts:
+    "C01RESEARCH": |
+      You are a research assistant. Focus on academic sources,
+      citations, and concise synthesis.
+    "C02ENGINEERING": |
+      Code review mode. Be precise about edge cases and
+      performance implications.
+```
+
+Keys are Slack channel IDs (find them via channel details → "About" → scroll to bottom). All messages in the matching channel get the prompt injected as an ephemeral system instruction.
+
 ## Troubleshooting
 
 | Problem | Solution |

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

@@ -526,6 +526,29 @@ Unlike Discord (where reactions are additive), Telegram's Bot API replaces all b
 If the bot doesn't have permission to add reactions in a group, the reaction calls fail silently and message processing continues normally.
 :::
 
+## Per-Channel Prompts
+
+Assign ephemeral system prompts to specific Telegram groups or forum topics. The prompt is injected at runtime on every turn — never persisted to transcript history — so changes take effect immediately.
+
+```yaml
+telegram:
+  channel_prompts:
+    "-1001234567890": |
+      You are a research assistant. Focus on academic sources,
+      citations, and concise synthesis.
+    "42":  |
+      This topic is for creative writing feedback. Be warm and
+      constructive.
+```
+
+Keys are chat IDs (groups/supergroups) or forum topic IDs. For forum groups, topic-level prompts override the group-level prompt:
+
+- Message in topic `42` inside group `-1001234567890` → uses topic `42`'s prompt
+- Message in topic `99` (no explicit entry) → falls back to group `-1001234567890`'s prompt
+- Message in a group with no entry → no channel prompt applied
+
+Numeric YAML keys are automatically normalized to strings.
+
 ## Troubleshooting
 
 | Problem | Solution |