fix(exec): apply per-agent exec defaults for opaque session keys

Co-authored-by: brin-tapcart <brin-tapcart@users.noreply.github.com>

src/agents/pi-embedded-runner.resolvesessionagentids.test.ts

@@ -48,4 +48,21 @@ describe("resolveSessionAgentIds", () => {
     });
     expect(sessionAgentId).toBe("main");
   });
+
+  it("uses explicit agentId when sessionKey is missing", () => {
+    const { sessionAgentId } = resolveSessionAgentIds({
+      agentId: "main",
+      config: cfg,
+    });
+    expect(sessionAgentId).toBe("main");
+  });
+
+  it("prefers explicit agentId over non-agent session keys", () => {
+    const { sessionAgentId } = resolveSessionAgentIds({
+      sessionKey: "telegram:slash:123",
+      agentId: "main",
+      config: cfg,
+    });
+    expect(sessionAgentId).toBe("main");
+  });
 });