feat(fallback_models): complete init-time and runtime integration

Implement full fallback_models support across all integration points:

1. Model Resolution Pipeline (src/shared/model-resolution-pipeline.ts)
   - Add userFallbackModels to ModelResolutionRequest
   - Process user fallback_models before hardcoded fallback chain
   - Support both connected provider and availability checking modes

2. Agent Utils (src/agents/utils.ts)
   - Update applyModelResolution to accept userFallbackModels
   - Inject fallback_models for all builtin agents (sisyphus, oracle, etc.)
   - Support both single string and array formats

3. Model Resolver (src/shared/model-resolver.ts)
   - Add userFallbackModels to ExtendedModelResolutionInput type
   - Pass through to resolveModelPipeline

4. Delegate Task Executor (src/tools/delegate-task/executor.ts)
   - Extract category fallback_models configuration
   - Pass to model resolution pipeline
   - Register session category for runtime-fallback hook

5. Session Category Registry (src/shared/session-category-registry.ts)
   - New module: maps sessionID -> category
   - Used by runtime-fallback to lookup category fallback_models
   - Auto-cleanup support

6. Runtime Fallback Hook (src/hooks/runtime-fallback/index.ts)
   - Check SessionCategoryRegistry first for category fallback_models
   - Fallback to agent-level configuration
   - Import and use SessionCategoryRegistry

Test Results:
- runtime-fallback: 24/24 tests passing
- model-resolver: 46/46 tests passing

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

src/hooks/runtime-fallback/index.ts

@@ -3,6 +3,7 @@ import type { RuntimeFallbackConfig, OhMyOpenCodeConfig } from "../../config"
 import type { FallbackState, FallbackResult, RuntimeFallbackHook } from "./types"
 import { DEFAULT_CONFIG, RETRYABLE_ERROR_PATTERNS, HOOK_NAME } from "./constants"
 import { log } from "../../shared/logger"
+import { SessionCategoryRegistry } from "../../shared/session-category-registry"
 
 function createFallbackState(originalModel: string): FallbackState {
   return {
@@ -87,6 +88,15 @@ function getFallbackModelsForSession(
 ): string[] {
   if (!pluginConfig) return []
 
+  //#when - session has category from delegate_task, try category fallback_models first
+  const sessionCategory = SessionCategoryRegistry.get(sessionID)
+  if (sessionCategory && pluginConfig.categories?.[sessionCategory]) {
+    const categoryConfig = pluginConfig.categories[sessionCategory]
+    if (categoryConfig?.fallback_models) {
+      return normalizeFallbackModels(categoryConfig.fallback_models)
+    }
+  }
+
   const tryGetFallbackFromAgent = (agentName: string): string[] | undefined => {
     const agentConfig = pluginConfig.agents?.[agentName as keyof typeof pluginConfig.agents]
     if (!agentConfig) return undefined