feat(config): add runtime_fallback and fallback_models schema

Add configuration schemas for runtime model fallback feature:
- RuntimeFallbackConfigSchema with enabled, retry_on_errors,
  max_fallback_attempts, cooldown_seconds, notify_on_fallback
- FallbackModelsSchema for init-time fallback model selection
- Add fallback_models to AgentOverrideConfigSchema and CategoryConfigSchema
- Export types and schemas from config/index.ts

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/runtime-fallback/constants.ts

@@ -0,0 +1,40 @@
+/**
+ * Runtime Fallback Hook - Constants
+ *
+ * Default values and configuration constants for the runtime fallback feature.
+ */
+
+import type { RuntimeFallbackConfig } from "../../config"
+
+/**
+ * Default configuration values for runtime fallback
+ */
+export const DEFAULT_CONFIG: Required<RuntimeFallbackConfig> = {
+  enabled: true,
+  retry_on_errors: [429, 503, 529],
+  max_fallback_attempts: 3,
+  cooldown_seconds: 60,
+  notify_on_fallback: true,
+}
+
+/**
+ * Error patterns that indicate rate limiting or temporary failures
+ * These are checked in addition to HTTP status codes
+ */
+export const RETRYABLE_ERROR_PATTERNS = [
+  /rate.?limit/i,
+  /too.?many.?requests/i,
+  /quota.?exceeded/i,
+  /service.?unavailable/i,
+  /overloaded/i,
+  /temporarily.?unavailable/i,
+  /try.?again/i,
+  /429/,
+  /503/,
+  /529/,
+]
+
+/**
+ * Hook name for identification and logging
+ */
+export const HOOK_NAME = "runtime-fallback"