fix: add sessionWriteLockTimeoutMs to AgentDefaultsConfig type

Adds the missing TypeScript type definition for the sessionWriteLockTimeoutMs
config option, fixing the build error (TS2339).

Part of #4355

src/config/types.agent-defaults.ts

@@ -196,6 +196,8 @@ export type AgentDefaultsConfig = {
   };
   /** Max concurrent agent runs across all conversations. Default: 1 (sequential). */
   maxConcurrent?: number;
+  /** Timeout (ms) for acquiring per-session write lock. Default: 10000. */
+  sessionWriteLockTimeoutMs?: number;
   /** Sub-agent defaults (spawned via sessions_spawn). */
   subagents?: {
     /** Max concurrent sub-agent runs (global lane: "subagent"). Default: 1. */