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/sisyphus-agent.ts

@@ -52,7 +52,7 @@ export function maybeCreateSisyphusConfig(input: {
   if (disabledAgents.includes("sisyphus") || !meetsSisyphusAnyModelRequirement) return undefined
 
   let sisyphusResolution = applyModelResolution({
-    uiSelectedModel: sisyphusOverride?.model ? undefined : uiSelectedModel,
+    uiSelectedModel: sisyphusOverride?.model !== undefined ? undefined : uiSelectedModel,
     userModel: sisyphusOverride?.model,
     requirement: sisyphusRequirement,
     availableModels,