build: native subagent pipeline + Codex-only asset producer

Adds an agent cross-compile pipeline alongside the existing skill
pipeline. Sources live at skill/agents/*.md; providers that declare
agentFormat (codex-toml, claude-md) emit native subagent files. An
optional providers: <list> field on an agent gates which harnesses
get a copy; default (no field) ships everywhere.

The impeccable-asset-producer agent is opt-in to Codex only. It's
useful for Codex's native image generation path and is untested
elsewhere; Claude has no native image gen anyway.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/lib/transformers/providers.js

@@ -20,6 +20,7 @@ export const PROVIDERS = {
     configDir: '.claude',
     displayName: 'Claude Code',
     frontmatterFields: ['user-invocable', 'argument-hint', 'license', 'compatibility', 'metadata', 'allowed-tools'],
+    agentFormat: 'claude-md',
   },
   gemini: {
     provider: 'gemini',
@@ -34,6 +35,7 @@ export const PROVIDERS = {
     frontmatterFields: [],
     includeVersion: false,
     writeOpenAIMetadata: true,
+    agentFormat: 'codex-toml',
   },
   agents: {
     provider: 'agents',