fix(agents): preserve uiSelectedModel when agent override has no model

Three agent builder files used falsy checks that incorrectly nullified
uiSelectedModel when override objects existed but had no model set:

- sisyphus-agent.ts: `?.model ?` → `?.model !== undefined ?`
- atlas-agent.ts: `?.model ?` → `?.model !== undefined ?`
- general-agents.ts: `!override?.model` → `override?.model === undefined`

This caused user model selection in web mode to revert to defaults.

Fixes #2351

src/agents/builtin-agents/general-agents.ts

@@ -69,7 +69,7 @@ export function collectPendingBuiltinAgents(input: {
     const isPrimaryAgent = isFactory(source) && source.mode === "primary"
 
     let resolution = applyModelResolution({
-      uiSelectedModel: (isPrimaryAgent && !override?.model) ? uiSelectedModel : undefined,
+      uiSelectedModel: (isPrimaryAgent && override?.model === undefined) ? uiSelectedModel : undefined,
       userModel: override?.model,
       requirement,
       availableModels,