[codex] align system test workflow with CI

[k82cn]

Summary

• align the system test workflow with the existing Flame CI workflow format
• run it on push and pull_request instead of a daily schedule
• remove the custom dispatch/concurrency shape so it behaves like the other CI jobs

Verification

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/e2e-system.yaml"); puts "ok"'
• git diff --check -- .github/workflows/e2e-system.yaml

[gemini-code-assist]

Note

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