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/shared/model-resolution-pipeline.ts

@@ -7,6 +7,7 @@ export type ModelResolutionRequest = {
   intent?: {
     uiSelectedModel?: string
     userModel?: string
+    userFallbackModels?: string[]
     categoryDefaultModel?: string
   }
   constraints: {
@@ -97,6 +98,42 @@ export function resolveModelPipeline(
     })
   }
 
+  //#when - user configured fallback_models, try them before hardcoded fallback chain
+  const userFallbackModels = intent?.userFallbackModels
+  if (userFallbackModels && userFallbackModels.length > 0) {
+    if (availableModels.size === 0) {
+      const connectedProviders = readConnectedProvidersCache()
+      const connectedSet = connectedProviders ? new Set(connectedProviders) : null
+
+      if (connectedSet !== null) {
+        for (const model of userFallbackModels) {
+          attempted.push(model)
+          const parts = model.split("/")
+          if (parts.length >= 2) {
+            const provider = parts[0]
+            if (connectedSet.has(provider)) {
+              log("Model resolved via user fallback_models (connected provider)", { model })
+              return { model, provenance: "provider-fallback", attempted }
+            }
+          }
+        }
+        log("No connected provider found in user fallback_models, falling through to hardcoded chain")
+      }
+    } else {
+      for (const model of userFallbackModels) {
+        attempted.push(model)
+        const parts = model.split("/")
+        const providerHint = parts.length >= 2 ? [parts[0]] : undefined
+        const match = fuzzyMatchModel(model, availableModels, providerHint)
+        if (match) {
+          log("Model resolved via user fallback_models (availability confirmed)", { model: model, match })
+          return { model: match, provenance: "provider-fallback", attempted }
+        }
+      }
+      log("No available model found in user fallback_models, falling through to hardcoded chain")
+    }
+  }
+
   if (fallbackChain && fallbackChain.length > 0) {
     if (availableModels.size === 0) {
       const connectedProviders = constraints.connectedProviders ?? connectedProvidersCache.readConnectedProvidersCache()