fix: respect agent model selection in heartbeat and manual runs

Heartbeat read non-existent split modelProvider/modelId fields while the
dashboard saves runtimeConfig.model as combined "provider/modelId", so
sessions fell through to pi's default model and failed with
"No API key for provider: openai-codex". Add extractRuntimeModel helper
that prefers the combined string and use it from heartbeat.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.changeset/heartbeat-agent-model.md

@@ -0,0 +1,5 @@
+---
+"@fusion/engine": patch
+---
+
+Fix heartbeat and manual agent runs ignoring the agent's configured model. The dashboard saves `runtimeConfig.model` as a combined `"provider/modelId"` string, but heartbeat was reading non-existent split `modelProvider`/`modelId` fields, causing sessions to fall through to pi's default model (often `openai-codex`) and fail with "No API key for provider: openai-codex".

packages/engine/src/agent-heartbeat.ts

@@ -1344,7 +1344,7 @@ export class HeartbeatMonitor {
           },
         };
 
-        const { createResolvedAgentSession, extractRuntimeHint } = await import("./agent-session-helpers.js");
+        const { createResolvedAgentSession, extractRuntimeHint, extractRuntimeModel } = await import("./agent-session-helpers.js");
         const { buildSessionSkillContextSync } = await import("./session-skill-context.js");
 
         // Build tools with task creation tracking and run context for mutation correlation
@@ -1449,8 +1449,10 @@ export class HeartbeatMonitor {
           systemPrompt,
           tools: "readonly",
           customTools: heartbeatTools,
-          defaultProvider: agent.runtimeConfig?.modelProvider as string | undefined,
-          defaultModelId: agent.runtimeConfig?.modelId as string | undefined,
+          ...(() => {
+            const { provider, modelId } = extractRuntimeModel(agent.runtimeConfig);
+            return { defaultProvider: provider, defaultModelId: modelId };
+          })(),
           onText: (delta) => {
             outputLength += delta.length;
             appendStdoutExcerpt(delta);

packages/engine/src/agent-session-helpers.ts

@@ -70,6 +70,37 @@ export function extractRuntimeHint(
   return normalizedHint.length > 0 ? normalizedHint : undefined;
 }
 
+/**
+ * Extract the model provider and id from an agent's runtimeConfig.
+ *
+ * The dashboard's NewAgentDialog stores the agent's selected model as a
+ * single combined string `runtimeConfig.model = "provider/modelId"` (see
+ * register-chat-routes.ts which parses the same shape). Older code paths
+ * also looked at separate `modelProvider` / `modelId` fields. This helper
+ * accepts either shape, preferring the combined `model` string.
+ */
+export function extractRuntimeModel(
+  runtimeConfig: Record<string, unknown> | undefined,
+): { provider: string | undefined; modelId: string | undefined } {
+  const combined = typeof runtimeConfig?.model === "string" ? runtimeConfig.model.trim() : "";
+  if (combined) {
+    const slashIdx = combined.indexOf("/");
+    if (slashIdx > 0 && slashIdx < combined.length - 1) {
+      return {
+        provider: combined.slice(0, slashIdx).trim() || undefined,
+        modelId: combined.slice(slashIdx + 1).trim() || undefined,
+      };
+    }
+  }
+
+  const provider = typeof runtimeConfig?.modelProvider === "string" ? runtimeConfig.modelProvider.trim() : "";
+  const modelId = typeof runtimeConfig?.modelId === "string" ? runtimeConfig.modelId.trim() : "";
+  return {
+    provider: provider || undefined,
+    modelId: modelId || undefined,
+  };
+}
+
 /**
  * Create an agent session using runtime resolution.
  *