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/agents/utils.ts

@@ -11,7 +11,7 @@ import { createAtlasAgent, atlasPromptMetadata } from "./atlas"
 import { createMomusAgent, momusPromptMetadata } from "./momus"
 import { createHephaestusAgent } from "./hephaestus"
 import type { AvailableAgent, AvailableCategory, AvailableSkill } from "./dynamic-agent-prompt-builder"
-import { deepMerge, fetchAvailableModels, resolveModelPipeline, AGENT_MODEL_REQUIREMENTS, readConnectedProvidersCache, isModelAvailable, isAnyFallbackModelAvailable, migrateAgentConfig } from "../shared"
+import { deepMerge, fetchAvailableModels, resolveModelPipeline, AGENT_MODEL_REQUIREMENTS, readConnectedProvidersCache, isModelAvailable, isAnyFallbackModelAvailable, isAnyProviderConnected, migrateAgentConfig } from "../shared"
 import { DEFAULT_CATEGORIES, CATEGORY_DESCRIPTIONS } from "../tools/delegate-task/constants"
 import { resolveMultipleSkills } from "../features/opencode-skill-loader/skill-content"
 import { createBuiltinSkills } from "../features/builtin-skills"
@@ -394,13 +394,13 @@ export async function createBuiltinAgents(
     const hephaestusRequirement = AGENT_MODEL_REQUIREMENTS["hephaestus"]
     const hasHephaestusExplicitConfig = hephaestusOverride !== undefined
 
-    const hasRequiredModel =
-      !hephaestusRequirement?.requiresModel ||
+    const hasRequiredProvider =
+      !hephaestusRequirement?.requiresProvider ||
       hasHephaestusExplicitConfig ||
       isFirstRunNoCache ||
-      isAnyFallbackModelAvailable(hephaestusRequirement.fallbackChain, availableModels)
+      isAnyProviderConnected(hephaestusRequirement.requiresProvider, availableModels)
 
-    if (hasRequiredModel) {
+    if (hasRequiredProvider) {
       let hephaestusResolution = applyModelResolution({
         userModel: hephaestusOverride?.model,
         requirement: hephaestusRequirement,