Honor DAG command execution context

[GreyforgeLabs]

Summary

• choose the DAG task shell by platform before executing task commands
• use cmd.exe /C on Windows and keep sh -c on Unix-like systems
• parse task-level working_dir and run command tasks from that directory

Fixes PlatformNetwork/bounty-challenge#53151
Fixes PlatformNetwork/bounty-challenge#52613

Tests

• cargo fmt --package cortex-cli --check
• git diff --check
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo run -p cortex-cli --no-default-features --features cortex-tui --bin Cortex -- dag run --file /tmp/cortex-dag-shell-smoke.yaml --strategy sequential --quiet
• cargo run -p cortex-cli --no-default-features --features cortex-tui --bin Cortex -- dag run --file /tmp/cortex-dag-working-dir.yaml --strategy sequential --quiet
• tr -d "\r\n" < /tmp/cortex-dag-working-dir-output.txt # output: /tmp

[greptile-apps]

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[greptile-apps]

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