feat(codex): add multi-agent integration

[YangsonHung]

What does this PR do?

Adds first-class Codex multi-agent integration for The Agency.

This updates the conversion pipeline to generate Codex-compatible project-scoped role configs under .codex/, adds installer support for Codex projects, and documents the workflow in the main README plus integration docs.

Included

• scripts/convert.sh --tool codex
• scripts/install.sh --tool codex
• generated integrations/codex/config.toml
• generated integrations/codex/agents/*.toml role files
• new integrations/codex/README.md
• README and integrations index updates

Codex-specific behavior

• Enables [features] multi_agent = true
• Registers one [agents.<slug>] role per Agency agent
• Keeps role config minimal by only injecting developer_instructions, so Codex inherits the user's model/sandbox defaults
• Installs into project-local .codex/
• Does not overwrite an existing .codex/config.toml; instead writes .codex/agency-agents.snippet.toml for manual merge

Validation

• bash -n scripts/convert.sh && bash -n scripts/install.sh
• ./scripts/convert.sh --tool codex
• ./scripts/convert.sh --tool all
• parsed generated Codex TOML outputs with Python tomllib
• tested install.sh --tool codex with and without an existing .codex/config.toml

Agent Information (if adding/modifying an agent)

• Agent Name: N/A
• Category: integrations / tooling
• Specialty: Codex multi-agent conversion and installation

Checklist

• Follows the agent template structure from CONTRIBUTING.md
• Includes YAML frontmatter with name, description, color
• Has concrete code/template examples (for new agents)
• Tested in real scenarios
• Proofread and formatted correctly