feat: configurable system prompt for messaging-gateway chats

feat: configurable system prompt for messaging-gateway chats

Feature description

Conversations routed through messaging gateways (Discord, WhatsApp, Telegram)
use the same base system prompt as local chats. It would be useful for admins
to customize the persona/instructions for gateway conversations separately —
for example, shorter or more notification-style answers suited to chat apps.

Proposed solution

• A gateway.system_prompt config (editable in the Gateway settings tab) that replaces the base system prompt for conversations with a gateway bridge id in their meta.
• When the override is set, skip the base-template resolution but keep all augmentation (memory, AGENTS.md instructions, skills, context) so gateway chats still behave like full agent chats.
• Empty config = zero behavior change (default).

Additional context

Implemented on a fork; can share implementation notes.

Start by tracing how gateway bridge IDs are identified in conversation metadata, how the base system prompt is resolved, and where the Gateway settings tab reads configuration. Done means an optional gateway.system_prompt replaces only the base prompt for gateway chats, preserves memory, AGENTS.md, skills, and context augmentation, and leaves behavior unchanged when empty.