fix(cloud-agent): inject prContext into codex prompts

[tatoalo]

Problem

On Slack-originated follow-up runs the agent-server sets _meta.prContext on the prompt so the agent sees the PR review context. Claude's acp-to-sdk.ts prepends it as a text block; codex forwarded params unchanged to codex-acp and the context was silently dropped — follow-ups lost their review framing.

Changes

Prepend _meta.prContext to the prompt as a text block before forwarding to codex-acp. Keep the user_message_chunk broadcast on the original params so the injected context is not rendered as a user message in the UI.

How did you test this?

• New unit test asserting the forwarded prompt carries the prContext prefix and the broadcast shows only the real user turn.
• Full @posthog/agent suite green.

[tatoalo]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(cloud-agent): serialize concurrent codex prompts to fix usage accounting #1721
• fix(cloud-agent): propagate taskRunId through codex loadSession #1720
• fix(cloud-agent): inject prContext into codex prompts #1719 👈 (View in Graphite)
• fix(cloud-agent): broadcast user prompt on codex cloud runs #1718
• main

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