Deprecate wp-now in README and CLI output

[adamziel]

What it does

Displays a deprecation notice whenever wp-now runs and adds the same warning to the @wp-now/wp-now README.

Rationale

wp-now is no longer maintained and should point users to @wp-playground/cli. This is the package-side follow-up to the Playground migration docs.

Supersedes #378 with a smaller patch and without the placeholder code left in that PR.

Implementation

• Adds a plain stderr deprecation notice at runCli() startup.
• Points users to npx @wp-playground/cli@latest start.
• Links to the Playground CLI migration guide.
• Adds the same recommendation at the top of the wp-now README.

Testing instructions

• Ran git diff --check.
• Ran npx --yes prettier@2.6.2 --check packages/wp-now/README.md packages/wp-now/src/run-cli.ts.

I did not run the full test suite because this checkout does not have dependencies installed.