fix: align Codex agent templates with schema

[FutureUnreal]

Summary

• Use top-level \sandbox_mode\ in Codex agent templates
• Use top-level string \developer_instructions\ instead of a nested table
• Remove unsupported \eatures.multi_agent\ blocks from the agent templates

Verification

• \pnpm typecheck`n- Parsed all three Codex agent TOML templates with \smol-toml\ and verified \developer_instructions\ is a string

Note: \pnpm test\ currently fails on Windows in \installWorkflows.test.ts\ due to an existing CRLF assertion mismatch: expected \ ools: Read, Write, received \ ools: Read, Write\r.