fix(runtime-fallback): 9 critical bug fixes for auto-retry, agent preservation, and model override

Bug fixes:
1. extractStatusCode: handle nested data.statusCode (Anthropic error structure)
2. Error regex: relax credit.*balance.*too.*low pattern for multi-char gaps
3. Zod schema: bump max_fallback_attempts from 10 to 20 (config rejected silently)
4. getFallbackModelsForSession: fallback to sisyphus/any agent when session.error lacks agent
5. Model detection: derive model from agent config when session.error lacks model info
6. Auto-retry: resend last user message with fallback model via promptAsync
7. Persistent fallback: override model on every chat.message (not just pendingFallbackModel)
8. Manual model change: detect UI model changes and reset fallback state
9. Agent preservation: include agent in promptAsync body to prevent defaulting to sisyphus

Additional:
- Add sessionRetryInFlight guard to prevent double-retries
- Add resolveAgentForSession with 3-tier resolution (event → session memory → session ID)
- Add normalizeAgentName for display names like "Prometheus (Planner)" → "prometheus"
- Add resolveAgentForSessionFromContext to fetch agent from session messages
- Move AGENT_NAMES and agentPattern to module scope for reuse
- Register runtime-fallback hooks in event.ts and chat-message.ts
- Remove diagnostic debug logging from isRetryableError
- Add 400 to default retry_on_errors and credit/balance patterns to RETRYABLE_ERROR_PATTERNS

src/hooks/runtime-fallback/constants.ts

@@ -11,7 +11,7 @@ import type { RuntimeFallbackConfig } from "../../config"
  */
 export const DEFAULT_CONFIG: Required<RuntimeFallbackConfig> = {
   enabled: true,
-  retry_on_errors: [429, 503, 529],
+  retry_on_errors: [400, 429, 503, 529],
   max_fallback_attempts: 3,
   cooldown_seconds: 60,
   notify_on_fallback: true,
@@ -29,6 +29,8 @@ export const RETRYABLE_ERROR_PATTERNS = [
   /overloaded/i,
   /temporarily.?unavailable/i,
   /try.?again/i,
+  /credit.*balance.*too.*low/i,
+  /insufficient.?(?:credits?|funds?|balance)/i,
   /(?:^|\s)429(?:\s|$)/,
   /(?:^|\s)503(?:\s|$)/,
   /(?:^|\s)529(?:\s|$)/,