fix(agent): tell Slack app not to wrap links in bold

[joshsny]

Problem

The PostHog Slack app frequently emits messages where a URL is wrapped in bold, e.g. *<https://github.com/PostHog/posthog/pull/61933*>. In Slack mrkdwn, bold is a single asterisk pair, so the trailing * ends up attached to the URL — when opened in a browser the link includes the stray * and 404s.

Changes

Added a Slack formatting section to the Slack-origin identity system prompt in agent-server.ts (buildCloudSystemPrompt). It tells the agent:

• Never wrap a URL or link in bold (or any) formatting, with a concrete example of how *https://...* breaks.
• Use Slack's <url|text> syntax for labeled links, again with no surrounding formatting.
• Bold is a single asterisk pair (*text*), not double (**text**), and should only be used for short labels — never around links.

This guidance is only injected for Slack-origin runs, alongside the existing identity block.

How did you test this?

• pnpm --filter agent typecheck — passes.
• Added assertions to the existing identity-instructions tests in agent-server.test.ts verifying the new # Slack formatting section is present for Slack-origin prompts. Note: the agent-server.test.ts suite could not be executed in the sandbox because its beforeEach calls createTestRepo (git/env dependency) which fails here independently of this change; the added assertions are plain substring checks against the generated prompt string.

Automatic notifications

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

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