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/agent-resolver.ts

@@ -0,0 +1,54 @@
+import { getSessionAgent } from "../../features/claude-code-session-state"
+
+export const AGENT_NAMES = [
+  "sisyphus",
+  "oracle",
+  "librarian",
+  "explore",
+  "prometheus",
+  "atlas",
+  "metis",
+  "momus",
+  "hephaestus",
+  "sisyphus-junior",
+  "build",
+  "plan",
+  "multimodal-looker",
+]
+
+export const agentPattern = new RegExp(
+  `\\b(${AGENT_NAMES
+    .sort((a, b) => b.length - a.length)
+    .map((a) => a.replace(/-/g, "\\-"))
+    .join("|")})\\b`,
+  "i",
+)
+
+export function detectAgentFromSession(sessionID: string): string | undefined {
+  const match = sessionID.match(agentPattern)
+  if (match) {
+    return match[1].toLowerCase()
+  }
+  return undefined
+}
+
+export function normalizeAgentName(agent: string | undefined): string | undefined {
+  if (!agent) return undefined
+  const normalized = agent.toLowerCase().trim()
+  if (AGENT_NAMES.includes(normalized)) {
+    return normalized
+  }
+  const match = normalized.match(agentPattern)
+  if (match) {
+    return match[1].toLowerCase()
+  }
+  return undefined
+}
+
+export function resolveAgentForSession(sessionID: string, eventAgent?: string): string | undefined {
+  return (
+    normalizeAgentName(eventAgent) ??
+    normalizeAgentName(getSessionAgent(sessionID)) ??
+    detectAgentFromSession(sessionID)
+  )
+}