feat: add Codex agent conversion and install support (#362)

Adds Codex as a conversion/install target: each agent → `~/.codex/agents/<slug>.toml` with the three required Codex fields (name, description, developer_instructions).

Validated: all 184 agents generate valid, parseable TOML (incl. 21k-char agents with embedded code blocks) via the PR's TOML basic-string escaper. Matches OpenAI's documented custom-agent schema.

Thanks @yunuskilicdev.

integrations/codex/README.md

@@ -0,0 +1,79 @@
+# Codex Integration
+
+Converts all Agency agents into Codex custom agent TOML files. Each source
+agent becomes one standalone `.toml` file containing the minimal Codex-required
+fields: `name`, `description`, and `developer_instructions`.
+
+## Installation
+
+### Prerequisites
+
+- [Codex](https://developers.openai.com/codex/overview) installed
+
+### Convert And Install
+
+```bash
+# Generate integration files (required on fresh clone)
+./scripts/convert.sh --tool codex
+
+# Install agents
+./scripts/install.sh --tool codex
+```
+
+This copies generated agent files to `~/.codex/agents/`.
+
+## Generated Format
+
+Each generated file lives in:
+
+```text
+integrations/codex/agents/<slug>.toml
+```
+
+The mapping is intentionally minimal:
+
+- `name` is copied from the source frontmatter unchanged
+- `description` is copied from the source frontmatter unchanged
+- `developer_instructions` contains the full Markdown body unchanged
+
+Source-only metadata such as `color`, `emoji`, `vibe`, and other unsupported
+frontmatter fields are omitted.
+
+## Usage
+
+After installation, reference the custom agent by name in Codex:
+
+```text
+Use the Frontend Developer agent to review this component.
+```
+
+Codex uses the `name` field inside the TOML file as the source of truth, so the
+generated filename slug is only for filesystem safety.
+
+## Regenerate
+
+After modifying source agents:
+
+```bash
+./scripts/convert.sh --tool codex
+./scripts/install.sh --tool codex
+```
+
+## Troubleshooting
+
+### Codex integration not found
+
+Generate the Codex artifacts before installing:
+
+```bash
+./scripts/convert.sh --tool codex
+```
+
+### Codex not detected
+
+Make sure `codex` is in your PATH, or that `~/.codex/` already exists:
+
+```bash
+which codex
+codex --help
+```