feat(runtime-fallback): automatic model switching on API errors

Implements runtime model fallback that automatically switches to backup models
when the primary model encounters transient errors (rate limits, overload, etc.).

Features:
- runtime_fallback configuration with customizable error codes, cooldown, notifications
- Runtime fallback hook intercepts API errors (429, 503, 529)
- Support for fallback_models from agent/category configuration
- Session-state TTL and periodic cleanup to prevent memory leaks
- Robust agent name detection with explicit AGENT_NAMES array
- Session category registry for category-specific fallback lookup

Schema changes:
- Add RuntimeFallbackConfigSchema with enabled, retry_on_errors, max_fallback_attempts,
  cooldown_seconds, notify_on_fallback options
- Add fallback_models to AgentOverrideConfigSchema and CategoryConfigSchema
- Add runtime-fallback to HookNameSchema

Files added:
- src/hooks/runtime-fallback/index.ts - Main hook implementation
- src/hooks/runtime-fallback/types.ts - Type definitions
- src/hooks/runtime-fallback/constants.ts - Constants and defaults
- src/hooks/runtime-fallback/index.test.ts - Comprehensive tests
- src/config/schema/runtime-fallback.ts - Schema definition
- src/shared/session-category-registry.ts - Session category tracking

Files modified:
- src/hooks/index.ts - Export runtime-fallback hook
- src/plugin/hooks/create-session-hooks.ts - Register runtime-fallback hook
- src/config/schema.ts - Export runtime-fallback schema
- src/config/schema/oh-my-opencode-config.ts - Add runtime_fallback config
- src/config/schema/agent-overrides.ts - Add fallback_models to agent config
- src/config/schema/categories.ts - Add fallback_models to category config
- src/config/schema/hooks.ts - Add runtime-fallback to hook names
- src/shared/index.ts - Export session-category-registry
- docs/configurations.md - Add Runtime Fallback documentation
- docs/features.md - Add runtime-fallback to hooks list

Supersedes #1237, #1408
Closes #1408

src/plugin/hooks/create-session-hooks.ts

@@ -179,9 +179,11 @@ export function createSessionHooks(args: {
 
   const runtimeFallback = isHookEnabled("runtime-fallback")
     ? safeHook("runtime-fallback", () =>
-        createRuntimeFallbackHook(ctx, { config: pluginConfig.runtime_fallback }))
+        createRuntimeFallbackHook(ctx, {
+          config: pluginConfig.runtime_fallback,
+          pluginConfig,
+        }))
     : null
-
   return {
     contextWindowMonitor,
     preemptiveCompaction,