fix(agents): honor user atlas model when resolution returns undefined (#4255)

Atlas was the only agent that bailed early (returning undefined) when
applyModelResolution returned undefined, even if the user had explicitly
configured agents.atlas.model. The general-agents.ts path already handled
this case by falling back to the user's override; sisyphus has its own
explicit-config check. Atlas had neither, so under edge cases (cold
provider cache, no system default, empty availableModels) Atlas was
silently dropped and OpenCode used its built-in default — surfaced as
the hardcoded claude-sonnet-4-6 in the reported bug.

This aligns atlas-agent.ts with the same defensive pattern used in
general-agents.ts: when resolution fails but the user has an explicit
model override, honor it directly instead of dropping the agent.

Adds a regression test covering the scenario where availableModels is
empty AND systemDefaultModel is undefined AND no provider cache exists.

src/agents/builtin-agents/atlas-agent.ts

@@ -38,7 +38,7 @@ export function maybeCreateAtlasConfig(input: {
   const orchestratorOverride = agentOverrides["atlas"]
   const atlasRequirement = AGENT_MODEL_REQUIREMENTS["atlas"]
 
-  const atlasResolution = applyModelResolution({
+  let atlasResolution = applyModelResolution({
     uiSelectedModel: orchestratorOverride?.model !== undefined ? undefined : uiSelectedModel,
     userModel: orchestratorOverride?.model,
     requirement: atlasRequirement,
@@ -46,6 +46,12 @@ export function maybeCreateAtlasConfig(input: {
     systemDefaultModel,
   })
 
+  if (!atlasResolution && orchestratorOverride?.model) {
+    // User explicitly configured a model but resolution failed (e.g., cold cache, no system default).
+    // Honor the user's choice directly instead of dropping Atlas entirely.
+    atlasResolution = { model: orchestratorOverride.model, provenance: "override" as const }
+  }
+
   if (!atlasResolution) return undefined
   const { model: atlasModel, variant: atlasResolvedVariant } = atlasResolution