feat(agent): tell Slack-bound agent to echo labeled mention tokens

[VojtechBartos]

Problem

Slack only renders <@U_ID|displayname> (or bare <@U_ID>) as a real ping. When the agent writes plain @displayname text in its reply, Slack treats it as text — the user it's referring to doesn't get notified.

The PostHog/posthog companion PR (PostHog/posthog#63495) makes the backend deliver Slack's labeled wire form <@U_ID|displayname> in the message context the agent sees. This PR closes the loop on the agent side: tell it to copy that token verbatim into its reply.

Changes

Add a one-paragraph instruction to the Slack-origin system prompt (next to the existing # Response Style section):

# Mentioning users
When you want to ping a Slack user in your reply, copy their `<@U…|displayname>` token verbatim from the message context — Slack renders that as a real mention. Writing `@displayname` as plain text does NOT ping them.

Only added when getCloudInteractionOrigin() === "slack", so non-Slack interactions are unaffected.

How did you test this?

Manual: end-to-end test from a Slack thread where the user @-mentions someone. With backend PR + this prompt change in place, the agent's reply renders mentioned users as actual Slack pings. Without the prompt change, the agent tends to paraphrase to plain @name and the ping is lost.

No automated tests added — the change is in a string literal.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit cdb38ee.}

[greptile-apps]

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