chore(codex): update default model to gpt-5.6-luna

[takai]

Summary

Update the default model for the codex engine from gpt-5.4-mini to gpt-5.6-luna.

gpt-5.4-mini is no longer usable with a ChatGPT-account Codex login — the API rejects it with:

400 invalid_request_error: The 'gpt-5.4-mini' model is not supported when using Codex with a ChatGPT account.

Changed in three places:

• internal/config/config.go — built-in default args for the codex engine
• internal/app/app_test.go — corresponding test expectation
• scripts/release-check.sh — codex config used by the acceptance script

Test plan

• mise run test — all packages pass
• mise run lint — no issues (ran via pre-commit hook)
• mise run release-check — codex passes all 4 prompt presets (conventional, default, gitmoji, karma) with gpt-5.6-luna; claude passes 4/4
• gemini engine is currently failing in release-check for an unrelated reason (gemini CLI IneligibleTierError — free-tier Code Assist no longer supports this client). Tracked separately, not caused by this change.

https://claude.ai/code/session_011S1wPtNeaQmdUW7UGBPwn9