Add dual S3 and GCS upload GitHub Actions reference workflow

[Ayush7614]

Summary

• Add examples/github-actions/s3-gcs-upload-telemetry.yml for dual CI object-storage upload with AWS OIDC, GCP workload identity, both BEACON_CI_S3_* / BEACON_CI_GCS_* env vars, and upload: s3,gcs.
• Link the workflow from CI telemetry export docs, Claude Code CI docs, and cli/beacon/README.md.

Background

S3 and GCS each have dedicated reference workflows. Dual upload (upload: s3,gcs) is supported by action.yml and documented for the CLI, but had no complete copy-paste GitHub Actions example. No open issues or PRs target this CI example gap (endpoint dual object-storage docs in #298 are a different surface).

Test plan

• Verified examples/github-actions/s3-gcs-upload-telemetry.yml parses as valid YAML
• Confirmed inputs match action.yml (upload: s3,gcs, BEACON_CI_S3_*, BEACON_CI_GCS_*)
• Ran go test ./internal/ci/... -run 'Upload|S3|GCS|Gcs' in cli/beacon
• Maintainer can copy the workflow into a test repo, configure both cloud credentials/buckets, and run workflow_dispatch

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Note

Low Risk
Documentation and reference workflow only; no changes to Beacon CI upload logic or the composite action.

Overview
Adds a copy-paste reference workflow for CI telemetry that uploads runtime.jsonl to both S3 and GCS in one job (upload: s3,gcs), including AWS OIDC, GCP workload identity, gcloud setup, and the BEACON_CI_S3_* / BEACON_CI_GCS_* env vars.

Docs and examples now cross-link that workflow from ci-telemetry-exports, Claude Code CI runtime docs, cli/beacon/README.md, and comments in claude-code-telemetry.yml (including noting s3,gcs as a supported upload value). No runtime or action behavior changes—only documentation and the new example under examples/github-actions/.

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