channels.imessage.groups.<chat_id|*>.systemPrompt is now resolved at
inbound time and forwarded as ctxPayload.GroupSystemPrompt for group
messages, matching the established pattern in Discord, Telegram, IRC,
Slack, GoogleChat, and the retired BlueBubbles channel.
- Add 'systemPrompt?: string' to the channels.imessage.groups[*] entry
schema (types.imessage.ts + zod parser).
- Capture groupListPolicy.groupConfig.systemPrompt at decision build
time, falling back to the groups['*'] wildcard when the per-chat_id
entry has no explicit prompt. DM decisions never carry the prompt.
- Wire decision.groupSystemPrompt through to ctxPayload.GroupSystemPrompt
in buildIMessageInboundContext, gated on decision.isGroup.
Closes#78285.
b188d292bc