[codex] Add Modelsell provider preset

[modelsell]

Summary

• add Modelsell as a built-in API-key provider using the existing openai-chat adapter
• use the canonical https://modelsell.com/v1 endpoint and authenticated live model discovery
• expose Modelsell consistently in CLI/dashboard metadata, provider naming, tests, and localized docs

Closes #561.

Verification

• bun run typecheck
• bun test tests/modelsell-provider.test.ts tests/provider-registry-parity.test.ts (34 passed)
• bun run lint:gui
• bun run build:gui
• bun run privacy:scan
• git diff --check

The full bun run prepush suite was also attempted. It did not complete cleanly in this environment because unrelated existing child-process tests timed out and test-only hosts resolved to 198.18.x, which triggered destination-policy blocks. The Modelsell-focused tests and checks above pass.

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

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