fix: avoid github status workflow false positives

[Bortlesboat]

Fixes PlatformNetwork/bounty-challenge#43632.

Tightens cortex github status workflow detection so unrelated workflows that merely mention Cortex are not reported as installed. Detection now requires the generated Cortex workflow marker and the cortex github run command shape emitted by cortex github install.

Verification:

• cargo test -p cortex-cli --test github_status_workflow_detection -- --nocapture
• cargo check -p cortex-cli
• cargo fmt --check --package cortex-cli
• git diff --check

[greptile-apps]

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