feat(slack): forbid fabricating mention tokens

Tighten the Slack-origin system prompt rule on user mentions: require
that the `<@U…|displayname>` token come from the thread context
verbatim, and explicitly forbid substituting the display name (or any
other string) for the `U…` ID. Falls back to plain text when no
labeled token exists.

Follow-up to #2676: the agent learned the token's shape but invented
the user ID — observed in a Slack reply rendering `<@paul|Paul D'Ambra>`,
which Slack doesn't ping.

Author: VojtechBartos

packages/agent/src/server/agent-server.ts

@@ -1731,7 +1731,7 @@ You are replying in a Slack thread. Slack readers want short, skimmable answers
 - This is a default, not a hard rule. If the user (or their saved memory) asks for more depth or a specific format, follow that instead.
 
 # 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.
+To ping a Slack user, reuse a \`<@U…|displayname>\` token that already appears in the message context — copy it verbatim, including the \`U…\` ID. Do NOT construct a mention token from a name, and do NOT substitute the display name (or any other string) for the \`U…\` ID — \`<@Paul|Paul D'Ambra>\` is not a valid mention; only the form with the real ID like \`<@U02DXFTVDNE|Paul D'Ambra>\` is. If the person you want to refer to has no \`<@U…|displayname>\` token anywhere in the thread context, write their name as plain text instead of inventing one.
 
 # Suggesting code changes
 You can also open pull requests directly from this Slack thread. When the user's question describes a problem with a plausible code-side fix — a bug visible in errors or logs, missing or broken instrumentation, a broken funnel step traceable to UI code, a stale config that lives in a repo — end your reply with a one-sentence offer to open a PR for the fix and ask if they want you to proceed. Skip the offer for pure data lookups with no actionable code change (e.g. "what was DAU yesterday?"), and skip it when the fix would clearly live outside any repo you can reach.