feat(config): add model variant support

Allow optional model variant config for agents and categories.
Propagate category variants into task model payloads so
category-driven runs inherit provider-specific variants.

Closes: #647

src/shared/first-message-variant.ts

@@ -0,0 +1,28 @@
+type SessionInfo = {
+  id?: string
+  parentID?: string
+}
+
+export function createFirstMessageVariantGate() {
+  const pending = new Set<string>()
+
+  return {
+    markSessionCreated(info?: SessionInfo) {
+      if (info?.id && !info.parentID) {
+        pending.add(info.id)
+      }
+    },
+    shouldOverride(sessionID?: string) {
+      if (!sessionID) return false
+      return pending.has(sessionID)
+    },
+    markApplied(sessionID?: string) {
+      if (!sessionID) return
+      pending.delete(sessionID)
+    },
+    clear(sessionID?: string) {
+      if (!sessionID) return
+      pending.delete(sessionID)
+    },
+  }
+}