[codex] Support plugin agent roles

[charlesgong-openai]

Summary

• Add plugin manifest and default-directory support for bundled agent role TOML files.
• Load active plugin agent roles into per-turn config so spawn_agent can use namespaced agent_type values like sample:researcher.
• Update plugin creator validation, scaffolding, and manifest docs for the new agents component path.

Validation

• just fmt
• env CARGO_TARGET_DIR=/tmp/codex-target just test -p codex-core plugin_agent_roles_are_available_to_spawn_agent
• env CARGO_TARGET_DIR=/tmp/codex-target just test -p codex-core plugin_agent_roles_are_namespaced_with_plugin_manifest_name
• env CARGO_TARGET_DIR=/tmp/codex-target just test -p codex-plugin
• env CARGO_TARGET_DIR=/tmp/codex-target just test -p codex-utils-plugins
• env CARGO_TARGET_DIR=/tmp/codex-target just test -p codex-core-plugins
• env CARGO_TARGET_DIR=/tmp/codex-target just test -p codex-mcp-extension
• env CARGO_TARGET_DIR=/tmp/codex-target just fix -p codex-core -p codex-core-plugins -p codex-plugin -p codex-utils-plugins -p codex-mcp-extension
• git diff --check

Not run: full just test suite; it requires explicit approval because codex-core changed.