ci(os-132): remove obsolete shadow workflows

[jtoelke2]

Summary

Remove obsolete shadow GitHub Actions workflows now that the supported shared-runner paths are live. Keep the Rust native build helper as a reusable workflow, but drop its shadow/manual entrypoint and update Docker image builds to call the renamed workflow.

Related Issue

Linear OS-132 / OS-49 Phase 8

Changes

• Delete obsolete shadow-docker-build.yml and shadow-shared-cpu-spike.yml.
• Rename shadow-rust-native-build.yml to rust-native-build.yml.
• Keep rust-native-build.yml reusable-only via workflow_call.
• Update docker-build.yml to call the renamed Rust native build workflow.

Testing

• mise run pre-commit passes
  • Attempted locally; did not pass because markdownlint scanned untracked .codex-learning/ files and test:rust failed existing ssh::tests::launch_editor_returns_friendly_error_when_binary_missing.
• git diff --check origin/main...HEAD passed before push.
• Verified no remaining shadow-* workflow references before push.
• Parsed changed workflow YAML before push.
• Monitored post-merge Release Dev for ci(os-132): remove stale remote buildx mode #1267: build/package/release lanes passed, including Rust prebuilt jobs, local Buildx image builds, multi-arch manifests, Debian/RPM packages, Helm dev publish, and Release Dev upload. The overall run is red on the pre-existing user_namespaces E2E failure introduced before ci(os-132): remove stale remote buildx mode #1267.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)