fix(codex): avoid duplicate client instructions

[rexdotsh]

Summary

• promote a leading system/developer input into top-level Codex instructions when the client omits that field
• remove the promoted input item to match OpenCode native OAuth request shape
• retain explicit client instructions and use the vendored fallback only when no instruction context exists

Context

OpenCode sends its current GPT prompt as a developer input when using Kleis. Kleis previously also injected its vendored Codex fallback, causing gpt-5.5 and gpt-5.6-sol to receive two overlapping system prompts.

Verification

• bun test (86 pass)
• bun lint
• bun typecheck
• git diff --check