ci: deploy a preview of each labelled PR

[acoyfellow]

Deploys a preview of a PR so the team can test it before release, with real accounts and no local setup.

How it works

• A maintainer adds the preview label. The workflow deploys the PR head as cloudflare-api-mcp-pr-<n> on pr-<n>.<PREVIEW_DOMAIN_SUFFIX>, checks that /mcp answers 401 without a token, and comments the URL and a codex mcp add line on the PR.
• New commits remove the label, so each update needs a fresh review before it deploys again.
• Closing the PR deletes the Worker.
• The MCP host allowlist also accepts the HTTPS hostname in MCP_RESOURCE. Each deployment trusts only its own hostname. Production and staging behave as before.

Security

• pull_request_target runs PR code with the deploy secret, so deploys run only on the preview label, which only maintainers can add. npm ci --ignore-scripts keeps install scripts from running. The config script is read from the base branch, not the PR.
• Previews have no workers.dev or preview URLs. They hold no Cloudflare credential of their own: every call uses the tester's token.

Setup (one time, for maintainers)

• Secret PREVIEW_DEPLOY_TOKEN: Workers Scripts Edit and Workers Custom Domains Edit on the preview account.
• Variables: PREVIEW_ACCOUNT_ID, PREVIEW_DOMAIN_SUFFIX (a zone on that account), PREVIEW_KV_ID, PREVIEW_BUCKET (seeded with scripts/seed-r2.ts).
• Not included: OAuth sign-in. Testers use an API token until a preview OAuth client exists for pr-*.<suffix>.

Verified on acoyfellow/mcp

• Test PR preview workflow test (do not merge) acoyfellow/mcp#2 (feat(apps): render show_attention as an MCP App panel and sidebar entry #282's code), with the preview label added: run 38584100072 deployed https://pr-2.mcp-preview.ax.cloudflare.dev/mcp and commented the URL.
• Through that URL: initialize 200, tools/list includes show_attention with its UI resource, and show_attention returned the real items on a test account. Codex CLI called the tool through it.
• A push removed the label and posted the re-review comment.
• Closing the PR: run 38584302491 deleted the Worker (wrangler deployments list says it doesn't exist), and the URL stopped answering.
• npm test 486 passed, plus a new tests/mcp-deployed-host.test.ts. tsc, format:check, lint (1 warning, also on main) and actionlint pass.