Emit `codeguard-reviewer` subagent for Codex

[thomas-bartlett]

The codeguard-reviewer subagent currently only emits for Claude and
Cursor (see AGENT_HOSTS in src/artifact_targets.py). Codex users get
the CodeGuard skill but not the reviewer agent.

Codex loads subagents from .codex/agents/*.toml rather than Markdown,
so the current emitter in src/emit_agents.py can't produce it as-is.
The agent should reuse the existing software-security skill rather than
duplicating rule bodies into the TOML.

Test against a real Codex install:

• Agent is discovered (appears in the host's agent list/picker)
• Agent is invocable and runs to completion on a small repo
• Agent can read the rule files at the configured path
• SARIF output is produced

Docs: https://developers.openai.com/codex/subagents