Add Fastly deploy action with config push

[ChristianPavilonis]

Summary

• Add a Fastly-only EdgeZero deploy composite action at .github/actions/deploy.
• Add edgezero-cli config push --adapter fastly --file <path> -- --store-id <id> and Fastly Config Store upsert support.
• Document the GitHub Actions deploy contract, config-push behavior, and Trusted Server deployer migration plan.

Validation

• .github/actions/deploy/tests/run.sh
• bash -n .github/actions/deploy/scripts/*.sh .github/actions/deploy/tests/run.sh
• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.7
• zizmor --offline .github/actions/deploy/action.yml .github/workflows/deploy-action.yml
• cd docs && npm run format && npm run lint && npm run build
• cargo fmt --all -- --check
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo check --workspace --all-targets --features "fastly cloudflare"
• git diff --check

Notes

Use the commit SHA from this PR for pre-release action pinning while testing. If this PR changes before merge, update downstream pins to the latest desired commit SHA; for production, prefer the final merged commit SHA.