fix(agent-registration): always attempt fallback when model resolution fails

Removes both the isFirstRunNoCache and override?.model guards from
the fallback logic in collectPendingBuiltinAgents(). Previously, when
a user configured a model like minimax/MiniMax-M2.5 that wasn't in
availableModels, the agent was silently excluded and --agent Librarian
would crash with 'undefined is not an object'.

Now: if applyModelResolution() fails for ANY reason (cache state,
unavailable model, config merge issue), getFirstFallbackModel() is
always attempted. A log warning is emitted when a user-configured
model couldn't be resolved, making the previously silent failure
visible.

Supersedes #2517
Fixes #2427

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

@@ -8,6 +8,7 @@ import { buildAgent, isFactory } from "../agent-builder"
 import { applyOverrides } from "./agent-overrides"
 import { applyEnvironmentContext } from "./environment-context"
 import { applyModelResolution, getFirstFallbackModel } from "./model-resolution"
+import { log } from "../../shared/logger"
 
 export function collectPendingBuiltinAgents(input: {
   agentSources: Record<BuiltinAgentName, import("../agent-builder").AgentSource>
@@ -75,7 +76,13 @@ export function collectPendingBuiltinAgents(input: {
       availableModels,
       systemDefaultModel,
     })
-    if (!resolution && isFirstRunNoCache && !override?.model) {
+    if (!resolution) {
+      if (override?.model) {
+        log("[agent-registration] User-configured model could not be resolved, falling back", {
+          agent: agentName,
+          configuredModel: override.model,
+        })
+      }
       resolution = getFirstFallbackModel(requirement)
     }
     if (!resolution) continue