feat: check provider connectivity instead of specific model for hephaestus availability

Hephaestus now appears when any of its providers (openai, github-copilot, opencode) is
connected, rather than requiring the exact gpt-5.2-codex model. This allows users with
newer codex models (e.g., gpt-5.3-codex) to use Hephaestus without manual config overrides.

- Add requiresProvider field to ModelRequirement type
- Add isAnyProviderConnected() helper in model-availability
- Update hephaestus config from requiresModel to requiresProvider
- Update cli model-fallback to handle requiresProvider checks

src/cli/model-fallback.ts

@@ -122,6 +122,13 @@ function isRequiredModelAvailable(
   return matchingEntry.providers.some((provider) => isProviderAvailable(provider, avail))
 }
 
+function isRequiredProviderAvailable(
+  requiredProviders: string[],
+  avail: ProviderAvailability
+): boolean {
+  return requiredProviders.some((provider) => isProviderAvailable(provider, avail))
+}
+
 export function generateModelConfig(config: InstallConfig): GeneratedOmoConfig {
   const avail = toProviderAvailability(config)
   const hasAnyProvider =
@@ -185,6 +192,9 @@ export function generateModelConfig(config: InstallConfig): GeneratedOmoConfig {
     if (req.requiresModel && !isRequiredModelAvailable(req.requiresModel, req.fallbackChain, avail)) {
       continue
     }
+    if (req.requiresProvider && !isRequiredProviderAvailable(req.requiresProvider, avail)) {
+      continue
+    }
 
     const resolved = resolveModelFromChain(req.fallbackChain, avail)
     if (resolved) {
@@ -205,6 +215,9 @@ export function generateModelConfig(config: InstallConfig): GeneratedOmoConfig {
     if (req.requiresModel && !isRequiredModelAvailable(req.requiresModel, req.fallbackChain, avail)) {
       continue
     }
+    if (req.requiresProvider && !isRequiredProviderAvailable(req.requiresProvider, avail)) {
+      continue
+    }
 
     const resolved = resolveModelFromChain(fallbackChain, avail)
     if (resolved) {