Add native Codex agents integration

[Heazer13]

Summary

• add a native Codex integration that generates TOML custom agents from the Agency Agents markdown sources
• add installer and validator scripts for Codex agent directories
• document Codex usage alongside existing integrations
• wire Codex into the top-level conversion and installation helpers

Verification

• python3 integrations/codex/scripts/generate_codex_agents.py --repo-root . --output-agents-dir /tmp/agency-codex-agents-verify/agents --metadata-dir /tmp/agency-codex-agents-verify/meta
• python3 integrations/codex/scripts/validate_codex_agents.py --agents-dir /tmp/agency-codex-agents-verify/agents --manifest /tmp/agency-codex-agents-verify/meta/manifest.json
• python3 integrations/codex/tests/test_generate_codex_agents.py
• git diff --check HEAD~1..HEAD
• actionable secret scan on added diff: 0 hits

[Mexflubber]

we already have #362

[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!