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/config/schema/runtime-fallback.ts

@@ -1,11 +1,16 @@
 import { z } from "zod"
 
 export const RuntimeFallbackConfigSchema = z.object({
-  enabled: z.boolean().default(true),
-  retry_on_errors: z.array(z.number()).default([429, 503, 529]),
-  max_fallback_attempts: z.number().min(1).max(10).default(3),
-  cooldown_seconds: z.number().min(0).default(60),
-  notify_on_fallback: z.boolean().default(true),
+  /** Enable runtime fallback (default: true) */
+  enabled: z.boolean().optional(),
+  /** HTTP status codes that trigger fallback (default: [429, 503, 529]) */
+  retry_on_errors: z.array(z.number()).optional(),
+  /** Maximum fallback attempts per session (default: 3) */
+  max_fallback_attempts: z.number().min(1).max(20).optional(),
+  /** Cooldown in seconds before retrying a failed model (default: 60) */
+  cooldown_seconds: z.number().min(0).optional(),
+  /** Show toast notification when switching to fallback model (default: true) */
+  notify_on_fallback: z.boolean().optional(),
 })
 
 export type RuntimeFallbackConfig = z.infer<typeof RuntimeFallbackConfigSchema>