[codex] harden command execution and tests

[lobis]

Summary

• make EOS client command execution honor caller contexts while preserving per-client timeouts
• add an injectable command runner for focused unit tests around command execution and cancellation
• make integration tests opt-in by default and enable them explicitly in CI
• preserve shell-exit status updates through the Bubble Tea message loop
• cache command history reads incrementally instead of rescanning the whole log on every panel tick
• align source-install docs with the Go version declared in go.mod

Testing

• go test ./...
• go build ./...
• go vet ./...
• go test -cover ./...
• go test -race ./...
• go test -run TestIntegration ./eos/... (verifies the integration suite skips unless opted in)