refactor(main): extract getOrCreateStore in RuntimeBridge

P3-56: three call sites repeated the same 'lookup → fall back to
new → set if missing' dance for session stores. The duplication hid the
fact that the fallback allocation in the second and third sites could
be wasted when the entry already existed. Extract a single
getOrCreateStore helper that does one map lookup, one conditional
construction, and always returns the cached instance.

src/renderer/components/communication/ChannelSidebar.tsx

@@ -6,12 +6,12 @@ import { AgentAvatar } from "./MessageItem"
 import type { ChannelDescriptor, TaskDiscussionDescriptor } from "./ChannelHeader"
 import { useT } from "../../i18n"
 
-const channelConfig: Record<MessageChannel, { label: string; icon: React.ElementType; color: string }> = {
-  direct: { label: "Direct Messages", icon: EnvelopeSimple, color: "var(--primary)" },
-  department: { label: "Department", icon: Buildings, color: "var(--muted-foreground)" },
-  company: { label: "Company-wide", icon: Megaphone, color: "var(--primary)" },
-  project: { label: "Project", icon: Hash, color: "var(--primary)" },
-  incident: { label: "Incident", icon: Fire, color: "var(--destructive)" },
+const channelConfig: Record<MessageChannel, { label: string; icon: React.ElementType }> = {
+  direct: { label: "Direct Messages", icon: EnvelopeSimple },
+  department: { label: "Department", icon: Buildings },
+  company: { label: "Company-wide", icon: Megaphone },
+  project: { label: "Project", icon: Hash },
+  incident: { label: "Incident", icon: Fire },
 }
 
 function ChannelItem({
@@ -45,11 +45,8 @@ function ChannelItem({
       {descriptor.channel === "direct" && agent ? (
         <AgentAvatar agent={agent} size="sm" />
       ) : (
-        <div
-          className="flex h-7 w-7 shrink-0 items-center justify-center rounded-lg"
-          style={{ backgroundColor: `${config.color}18` }}
-        >
-          <Icon size={14} weight={active ? "fill" : "regular"} style={{ color: config.color }} />
+        <div className="flex h-7 w-7 shrink-0 items-center justify-center rounded-lg bg-muted">
+          <Icon size={14} weight={active ? "fill" : "regular"} className="text-muted-foreground" />
         </div>
       )}