codex: Add support for installing plugins and marketplaces

Install the referenced plugins into $CONFIG_DIR/plugins and add them to
the marketplace file in `~/.agents/plugins/marketplace.json`. The
plugins are then enabled in `$CONFIG_DIR/config.toml`.

Also add support for custom plugin marketplaces via the `marketplaces`
option. Each marketplace entry is a local directory (or fetched package)
that Codex can browse to discover additional plugins. Marketplaces are
registered in `$CONFIG_DIR/config.toml` under [marketplaces.<name>] with
`source_type = "local"` pointing at the provided path. Setting either
`plugins` or `marketplaces` automatically enables the `features.plugins`
flag in the generated config.

Warn that if a derivation is being used as the source of the plugin, the
name of the derivation needs to match the plugin name in the manifest.

tests/modules/programs/codex/sample-plugin/.codex-plugin/plugin.json

@@ -0,0 +1,6 @@
+{
+  "name": "sample-plugin",
+  "version": "1.0.0",
+  "description": "Sample Codex plugin for Home Manager tests.",
+  "skills": "./skills"
+}

tests/modules/programs/codex/sample-plugin/skills/sample/SKILL.md

@@ -0,0 +1,8 @@
+---
+name: sample
+description: Sample skill for Home Manager tests.
+---
+
+# Sample
+
+Test fixture content.