fix(agent): Hyperlink pull requests in agent replies

[charlesvien]

Problem

Agents mention the PRs they open as plain text. The link users usually see on cloud tasks comes from workflow plumbing, not the agent's own reply, so worktree and local sessions surface PRs without a link. None of our prompts told the agent to hyperlink PRs. Companion to PostHog/posthog#68770, which fixes the repo-owned cloud task prompts.

Changes

Adds one instruction to every prompt surface that produces user-facing replies: when mentioning a pull request, hyperlink its full URL as a markdown link instead of plain text.

• APPENDED_INSTRUCTIONS in the Claude session adapter (every desktop session, including worktree tasks)
• buildCloudSystemPrompt in agent-server (Slack app plus cloud and inbox runs; shared prLinkInstructions const included in all five prompt branches)
• buildSystemPrompt in workspace-server's agent service (locally spawned task and channel sessions)

How did you test this?

turbo typecheck on @posthog/agent and @posthog/workspace-server passes, and biome check on the three changed files reports nothing new (one pre-existing warning elsewhere in agent-server.ts). Prompt-string change only, so no runtime flow to exercise beyond prompt assembly.

Automatic notifications

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

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 4556a09.}

[github-actions]

Purely additive system-prompt instruction changes across three agent files telling the model to hyperlink PR references. No logic, API contracts, data models, or security surfaces touched.