New before_prompt_build hook that appends a "next action: workflow_start
chat" hint to the system prompt whenever the agent's turn was triggered
by a message in a fabric channel.
If Meridian (`globalThis.__meridian.getChatJournalForChannel`) reports
an existing chat journal for this (agentId, channelId), the hint
includes `from="<journal-id>"` so the agent resumes the conversation
file instead of starting a fresh one each turn.
Activation:
- ctx.agentId AND ctx.channelId present
- ctx.messageProvider in {fabric, '' (empty/omitted by gateway)}
TODO(phase-2): once Fabric exposes per-channel type info (DM / group /
triage) via a cross-plugin API, narrow this to xType === 'dm' only.
Today we fire on any fabric channel — chat workflow is a no-op outside
DMs, so the false positives are just prompt-text noise.
Dedup via WeakSet keyed on the event object (same pattern as the
existing before-prompt-build hook) so each turn injects at most once
even when multiple harness call sites trigger the hook.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
