feat: cross-repo cloud preview dispatch on PRs

[khaliqgant]

Summary

• Adds .github/workflows/cloud-preview.yml that triggers ephemeral CF Worker preview deploys in the cloud repo when PRs are opened/updated
• Each PR gets its own isolated D1 database with fresh migrations
• Preview resources (D1, KV, worker) are cleaned up automatically on PR close

How it works

PR opened/synced → dispatches cloud/preview-relayfile.yml (action=deploy)
  → creates relayfile-pr-{N} D1 + KV
  → runs all migrations from zero
  → deploys relayfile-api-pr-{N} worker
  → smoke test + E2E
  → reports ✅/❌ commit status on this PR

PR closed → dispatches cleanup (action=cleanup)
  → deletes D1, KV, worker

Prerequisites

• CLOUD_DISPATCH_TOKEN secret (GitHub App installation token with actions:write on AgentWorkforce/cloud)

Test plan

• Open a test PR and verify the cloud repo workflow is triggered
• Verify commit status appears on the PR
• Verify cleanup runs on PR close

🤖 Generated with Claude Code

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.