Add Codex integration support

[sundafei228-collab]

Summary

• add Codex as a supported conversion and install target
• generate Codex custom agent TOML under integrations/codex/agents
• document Codex setup in the main README and integrations docs

Validation

• bash -n scripts/convert.sh scripts/install.sh
• ./scripts/convert.sh --tool codex
• ./scripts/convert.sh --tool all --parallel
• validated 184 generated Codex TOML files with Python tomllib
• CODEX_HOME=$(mktemp -d) ./scripts/install.sh --tool codex installed 184 agents

[bensl84]

Closing — this is my personal fork (bensl84/agency-agents). The canonical project lives at https://github.com/msitarzewski/agency-agents — please re-open your PR there. Thanks!