feat(cloud-agent): add server-side cloud prompt handling

[tatoalo]

Problem

The agent server needs to deserialize structured cloud prompts (with embedded attachments) and forward them to the ACP session.

Changes

• Add packages/agent/src/server/cloud-prompt.ts: normalizeCloudPromptContent wrapper using @posthog/shared codec
• Update agent-server.ts: handle structured ContentBlock[] content in user messages, use promptBlocksToText for plain text extraction
• Update schemas.ts: add originalName to userMessageParamsSchema, structured content array support
• Add tests for structured content handling and question relay

Stack: 4/4 — depends on #1516

[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

• feat(cloud-agent): add server-side cloud prompt handling #1517 👈 (View in Graphite)
• feat(cloud-agent): wire cloud attachments into task creation and session display #1516
• feat(cloud-agent): add cloud prompt builder and file persistence #1515
• chore(cloud-agent): extract shared utilities for cloud attachments #1514 : 1 other dependent PR (#1519 )
• main

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[tatoalo]

Recreating with clean branch names