fix: emit valid Codex agent TOML

[artsparkAI]

Human Summary

Codex was unable to run gsd-planner subagent with error agent type is currently not available.
I was surprised to not see an issue open for this because it completely broke GSD usage within codex for me. Its possible that this may have been a quirk of just my system.

Agent Summary

What happened

Generated Codex agent TOML files used TOML basic multiline strings for developer_instructions. Several GSD agents include raw backslashes in regexes, shell snippets, and fenced examples. In TOML basic strings those backslashes are escape sequences, so the generated .toml files became invalid and Codex left those agents unavailable at runtime.

What this changes

• switch generated Codex agent developer_instructions to TOML literal multiline strings
• preserve agent prompt content verbatim instead of forcing TOML escape parsing
• update the Codex config test to assert the new delimiter

Verification

• node --test tests/codex-config.test.cjs
• generated and parsed all gsd-*.toml files successfully with tomllib after the change

Impact

This makes Codex installs generate valid agent configs for planner, verifier, researcher, mapper, and integration-checker prompts that contain raw backslashes.