Add automated publish-to-bcr workflow

[mmorel-35]

Implements automated publication to Bazel Central Registry using the publish-to-bcr tooling. s2geometry is already in BCR but publication is manual.

Changes

• .bcr/src/ templates: metadata with maintainers, source archive config with strip_prefix pointing to src/, presubmit matching existing BCR entry
• .bcr/config.yml: Specifies moduleRoots: ["src"] since MODULE.bazel lives in src/ directory
• .bcr/README.md: Setup instructions for maintainers covering PAT creation, secret configuration, and troubleshooting
• .github/workflows/publish-to-bcr.yml: Triggers on release events, uses publish-to-bcr@v1.0.0 reusable workflow with attestation support (no registry_fork parameter - auto-detects from PAT owner's fork)

Maintainer Setup Required

1. Create Classic PAT with repo and workflow permissions
2. PAT owner must have a fork of bazelbuild/bazel-central-registry
3. Add PAT as BCR_PUBLISH_TOKEN repository secret

After setup, new releases automatically create BCR pull requests. Manual trigger available via workflow_dispatch.

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[jmr]

@vtsao-openai

[jmr]

Looks reasonable. I need to learn more about this BCR workflow and become a maintainer of the BCR package.