refactor: rename gpt-5-4-prompting skill to codex-prompting

[tonyyunyang]

Summary

• Rename the gpt-5-4-prompting skill to codex-prompting so the label doesn't go stale as GPT iterates. The skill is really about prompting Codex (the runtime), not a specific GPT generation, and Codex already routes to whatever model the user has configured.
• Drop "GPT-5.4" mentions from the skill body and reference files. Update cross-references in agents/codex-rescue.md and skills/codex-cli-runtime/SKILL.md.
• Replace example gpt-5.4-mini strings in the README and the agent's model-passthrough rule with a <your-model> placeholder, since the actual model selection flows through ~/.codex/config.toml and --model.
• Update assertions in tests/commands.test.mjs to match.

Net-zero diff: 8 files, 21 insertions, 21 deletions. Folder rename detected by git (history preserved).

Test plan

• node --test tests/commands.test.mjs — all 8 assertions pass on the new branch
• No remaining gpt-5-4 / GPT-5.4 / gpt-5.4-mini strings in user-facing .md/.mjs/.json/.toml (verified via grep)
• Pre-existing failures in runtime.test.mjs and state.test.mjs confirmed unrelated — same 4 failures appear on baseline main when this branch's changes are stashed

Notes

• tests/fake-codex-fixture.mjs still has gpt-5.4 as a mock-server fallback. It's internal test infrastructure (not user-facing, not asserted on); left untouched to keep the diff minimal.
• The skill name has to be static — Claude Code resolves it from frontmatter at load time and can't reflect runtime config dynamically. Making it model-agnostic is the closest practical equivalent to "tied to whatever model the user has configured."