Fix: Validate sessionDefaults at config load time

Validate sessionDefaults immediately in readConfig instead of deferring
until writeMcpConfig. This fails fast on unknown keys or invalid value
types before expensive simulator setup, matching the documented behavior
in the benchmark README.

Author: cursoragent

CHANGELOG.md

@@ -22,6 +22,7 @@
 
 ### Fixed
 
+- Fixed Claude UI benchmark suite loading so `sessionDefaults` unknown keys and invalid value types fail fast at config load time instead of after expensive simulator setup.
 - Fixed Claude UI benchmark preflight so transient malformed or still-loading UI snapshots no longer crash the harness or finish before app UI is observable.
 - Fixed Claude UI benchmark preflight so configured first-run dismissals require a concrete simulator ID and suite-provided simulator IDs are recorded in command logs.
 - Fixed Claude UI benchmark config handling so invalid `failurePatterns` regexes fail before a suite starts and partial `allowedVariance` overrides preserve defaults for omitted metrics.

src/benchmarks/claude-ui/config.ts

@@ -185,7 +185,9 @@ export function readConfig(raw: unknown, source: string): BenchmarkConfig {
     ),
   };
 
-  if (isRecord(raw.sessionDefaults)) config.sessionDefaults = raw.sessionDefaults;
+  if (isRecord(raw.sessionDefaults)) {
+    config.sessionDefaults = validateSessionDefaults(raw.sessionDefaults);
+  }
   config.allowedVariance = readAllowedVariance(raw.allowedVariance, `${source}.allowedVariance`);
 
   if (raw.baseline !== undefined) {