fix(codex): resolve Python before JSON parsing

[jbetala7]

Summary

• Resolve python3 or python before the Codex JSONL parser runs in challenge and consult modes
• Fail with a clear error when no Python interpreter is available instead of relying on a hardcoded python3
• Regenerate codex/SKILL.md and add regression coverage for the portable parser command shape

Fixes #1303

Testing

• bun test test/skill-validation.test.ts -t "codex JSON stream parser uses portable Python discovery"
• bun run gen:skill-docs --host all --dry-run
• bun test test/skill-validation.test.ts

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