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.).
0d05bd34f8