refactor(runtime-fallback): decompose index.ts into focused modules

Split 1021-line index.ts into 10 focused modules per project conventions.

New structure:

- error-classifier.ts: error analysis with dynamic status code extraction

- agent-resolver.ts: agent detection utilities

- fallback-state.ts: state management and cooldown logic

- fallback-models.ts: model resolution from config

- auto-retry.ts: retry helpers with mutual recursion support

- event-handler.ts: session lifecycle events

- message-update-handler.ts: message.updated event handling

- chat-message-handler.ts: chat message interception

- hook.ts: main factory with proper cleanup

- types.ts: updated with HookDeps interface

- index.ts: 2-line barrel re-export

Embedded fixes:

- Fix setInterval leak with .unref()

- Replace require() with ESM import

- Add log warning on invalid model format

- Update sessionLastAccess on normal traffic

- Make extractStatusCode dynamic from config

- Remove unused SessionErrorInfo type

All 61 tests pass without modification.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/runtime-fallback/fallback-models.ts

@@ -0,0 +1,69 @@
+import type { OhMyOpenCodeConfig } from "../../config"
+import { AGENT_NAMES, agentPattern } from "./agent-resolver"
+import { HOOK_NAME } from "./constants"
+import { log } from "../../shared/logger"
+import { SessionCategoryRegistry } from "../../shared/session-category-registry"
+import { normalizeFallbackModels } from "../../shared/model-resolver"
+
+export function getFallbackModelsForSession(
+  sessionID: string,
+  agent: string | undefined,
+  pluginConfig: OhMyOpenCodeConfig | undefined
+): string[] {
+  if (!pluginConfig) return []
+
+  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
+    
+    if (agentConfig?.fallback_models) {
+      return normalizeFallbackModels(agentConfig.fallback_models)
+    }
+    
+    const agentCategory = agentConfig?.category
+    if (agentCategory && pluginConfig.categories?.[agentCategory]) {
+      const categoryConfig = pluginConfig.categories[agentCategory]
+      if (categoryConfig?.fallback_models) {
+        return normalizeFallbackModels(categoryConfig.fallback_models)
+      }
+    }
+    
+    return undefined
+  }
+
+  if (agent) {
+    const result = tryGetFallbackFromAgent(agent)
+    if (result) return result
+  }
+
+  const sessionAgentMatch = sessionID.match(agentPattern)
+  if (sessionAgentMatch) {
+    const detectedAgent = sessionAgentMatch[1].toLowerCase()
+    const result = tryGetFallbackFromAgent(detectedAgent)
+    if (result) return result
+  }
+
+  const sisyphusFallback = tryGetFallbackFromAgent("sisyphus")
+  if (sisyphusFallback) {
+    log(`[${HOOK_NAME}] Using sisyphus fallback models (no agent detected)`, { sessionID })
+    return sisyphusFallback
+  }
+
+  for (const agentName of AGENT_NAMES) {
+    const result = tryGetFallbackFromAgent(agentName)
+    if (result) {
+      log(`[${HOOK_NAME}] Using ${agentName} fallback models (no agent detected)`, { sessionID })
+      return result
+    }
+  }
+
+  return []
+}