Restrict GitHub workflow uninstall detection

[GreyforgeLabs]

Summary

• Require the cortex github install generator marker before github uninstall treats a workflow file as Cortex-owned.
• Prevent unrelated workflow files that merely mention cortex from being removed.
• Add binary integration coverage for rejecting an unrelated workflow and still removing a generated Cortex workflow.

Fixes PlatformNetwork/bounty-challenge#46726.

Validation

• cargo test -p cortex-cli --test github_uninstall_workflow --no-default-features --features cortex-tui -- --nocapture
• cargo check -p cortex-cli --no-default-features --features cortex-tui
• cargo fmt --package cortex-cli --check
• git diff --check

[greptile-apps]

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