fix(cloud-agent): broadcast user prompt on codex cloud runs

[tatoalo]

Problem

Cloud tasks running on the codex adapter don't display the user's message in the PostHog web UI log. codex-acp doesn't echo session/prompt back as a session/update, so the tapped stream persisted to S3 never sees a user turn and only the agent reply shows up.

Regressed by #1644 (first time codex became a cloud adapter — Claude had always broadcast).

Changes

Mirror ClaudeAcpAgent.broadcastUserMessage in CodexAcpAgent.prompt: emit one user_message_chunk session update per prompt chunk before delegating to codexConnection.prompt.

How did you test this?

• New unit test in codex-agent.test.ts asserting client.sessionUpdate is called with user_message_chunk for each chunk before codexConnection.prompt resolves.
• Full @posthog/agent suite green.

[tatoalo]

• 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
• fix(cloud-agent): broadcast user prompt on codex cloud runs #1718 👈 (View in Graphite)
• main

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