Fix Warden workflow: switch to OpenRouter and version tag

[dcramer]

The Warden sweep workflow is misconfigured in two ways:

1. It still uses an Anthropic API key (secrets.WARDEN_ANTHROPIC_API_KEY) instead of our OpenRouter bindings. See the .github repo for the correct setup.
2. It pins the getsentry/warden action to a specific git SHA (2130c979dec0163048d954d9599504e2d9fa2b07) instead of versioning to v0.

[sentry-pierre]

I cleaned the wording up so the two fixes are easier to scan.

Confirmed in the current checkout:

• .github/workflows/warden-sweep.yml pins getsentry/warden@2130c979dec0163048d954d9599504e2d9fa2b07.
• The same file passes anthropic-api-key: ${{ secrets.WARDEN_ANTHROPIC_API_KEY }}.

A maintainer can take it from here.

[cameroncooke]

@dcramer 🤷‍♂️