Fix god command honoring explicit provider

[garedrag]

Summary\n\n- Ensure an explicit agent provider selects its matching CLI binary when building spawn commands.\n- Prevent a Claude god configured alongside a global Codex default from launching as Codex with Claude-only flags.\n- Preserve custom commands and configured Claude commands that are actually Claude binaries.\n- Add a regression test for defaultCommand=codex with provider=claude.\n\n## Verification\n\n-

ode --test test/provider-config.test.cjs (9 passing)\n- git diff --check (passing)

[github-actions]

🚫 This PR is missing its before/after evidence

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How to fix it: edit the description, keep the ### Before and ### After headings from the template, and drag an image or video under each. GitHub uploads it inline. This check re-runs the moment you save.

A bug fix with no visible surface still needs it: show the failing behaviour, then the same steps passing. A terminal recording is fine.

Genuinely nothing to show — a CI tweak, a typo, a dependency bump? A maintainer can apply the no-visual-change label. Please don't ask unless it truly has no observable effect.

📖 CONTRIBUTING.md → Evidence is mandatory