This adds a new step for the `release` event to publish to the Official
MCP Registry.

Summary of changes:
- Add `server.json` which is the MCP Server manifest
  - Used metadata from the GH MCP Registry
  - Used the name `com.microsoft/playwright-mcp`
- Add `publish-release-mcp-registry` job that runs manually
   - Install Microsoft Go for compliant Go crypto
   - Log in to Azure for Key Vault access
   - Publish to the MCP Registry in the `microsoft.com` namespace.
- Add `mcpName` to the `package.json` which the MCP Registry validates.

The publishing requires some secret variables:
- `AZURE_MCP_REGISTRY_CLIENT_ID` - service principal client ID (app ID)
with Get Key and Sign access to a Key Vault
- `AZURE_MCP_REGISTRY_TENANT_ID` - tenant ID for the service principal
(should be a production tenant)
- `AZURE_MCP_REGISTRY_SUBSCRIPTION_ID` - Azure subscription ID holding
the Key Vault
- `KV_NAME` - the Key Vault name, e.g. `mykeyvault`
- `KV_KEY_NAME` - the name of a Key in the Key Vault, needs to be ECDSA
P-384.

I can provide internal documentation for how to set up the service
principal (managed identity is easiest), vault, and key as well as
registering the key so it can be used for the `microsoft.com` namespace.

**Note:** after the new job is proven to work as expected, we can switch
it to automatically running after the `publish-release-npm` job on the
`release` event.

…osoft#1289)

## Summary
- Adds `server.json` declaring the npm and OCI packages under the
`io.github.microsoft/playwright-mcp` namespace (already claimed via
`mcpName` in `package.json`).
- Adds a `publish-mcp-release-registry` job to `publish.yml` that runs
after the npm release, auto-syncs `server.json`'s version from
`package.json`, and publishes via `mcp-publisher` using GitHub OIDC (no
PAT needed).
- Unblocks enterprise users on `registryOnly` MCP policies (e.g. VS Code
+ Copilot in security-hardened setups) by listing `@playwright/mcp` in
the official registry.

Fixes #1477

## Precedent

[microsoft/mcp-dotnet-samples](https://github.com/microsoft/mcp-dotnet-samples/blob/main/.github/workflows/build-container.yaml)
publishes `io.github.microsoft/awesome-copilot` to the registry from an
hourly cron using the exact same flow (`id-token: write` + curl-fetched
`mcp-publisher` + `login github-oidc`). It's been running successfully
for weeks, so the GitHub-OIDC path is confirmed to work under
`microsoft/*` corp policy — no DNS / Key Vault setup needed (unlike the
reverted #1197 attempt that used the `com.microsoft/...` namespace).

## Notes
- `server.json`'s committed version is just a placeholder; the workflow
rewrites it from `package.json` before publish, so future `chore: mark
vX.Y.Z` commits don't need to touch `server.json`.
- First release after merge is the live test — no dry-run path.