feat(clients): configure Codex Desktop with guarded user settings

[neurogen-dev]

Problem

The public installer configured only separate CLI launchers. Codex Desktop reads the user-level config and continued to use OpenAI even after CLI setup.

Changes

• Offer an explicit Codex Desktop option on Windows and macOS.
• Write a guarded user-level provider using codex.neuroapi.host, HTTP/SSE, protected DPAPI/Keychain authentication, and a key-scoped model catalog.
• Back up the original config, reject conflicts and concurrent edits, and retain the helper/key if the user changes the config before uninstall.
• Add Windows/macOS lifecycle tests and connection documentation. Claude Desktop remains configured through its supported in-app Third-Party Inference form.

Verification

• macOS syntax, ShellCheck, isolated smoke including rollback and uninstall: passed.
• PowerShell Desktop config and static tests: passed locally.
• Native Windows DPAPI and client checks run in this PR's Windows Actions job.
• A separate GUI test on Windows showed Codex Desktop creating an HTML file through the NeuroAPI provider and Claude Desktop on claude.neuroapi.host creating and rereading a file.

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