Migrate public CI to Buildkite hosted runners

[luca-ctx]

Summary

Migrate the public CI gate from GitHub Actions to Buildkite hosted Linux runners.

• Route the Buildkite public smoke gate to the hosted linux-small queue.
• Move the Ubuntu/Rust/Bazel bootstrap from the deleted GHA workflow into scripts/buildkite-public-ci.sh.
• Update the Buildkite pipeline guard so the public smoke step cannot drift back to self-hosted runner tags.
• Harden current CI blockers found while validating the full gate: LOC exceptions for current oversized files, Python bytecode cache isolation, and SwiftPM scratch isolation under Bazel.

Validation

• buildkite-agent pipeline upload --dry-run --no-interpolation --format yaml .buildkite/pipeline.yml
• bazel test //:buildkite_pipeline_check --config=ci
• bazel test //:loc_check //:sdk_package_dry_run //:sdk_contract_checks --config=ci
• bash scripts/check.sh --mode=ci — 27 passing, 0 failing