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.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, test } from "bun:test"
+import { createFirstMessageVariantGate } from "./first-message-variant"
+
+describe("createFirstMessageVariantGate", () => {
+  test("marks new sessions and clears after apply", () => {
+    // #given
+    const gate = createFirstMessageVariantGate()
+
+    // #when
+    gate.markSessionCreated({ id: "session-1" })
+
+    // #then
+    expect(gate.shouldOverride("session-1")).toBe(true)
+
+    // #when
+    gate.markApplied("session-1")
+
+    // #then
+    expect(gate.shouldOverride("session-1")).toBe(false)
+  })
+
+  test("ignores forked sessions", () => {
+    // #given
+    const gate = createFirstMessageVariantGate()
+
+    // #when
+    gate.markSessionCreated({ id: "session-2", parentID: "session-parent" })
+
+    // #then
+    expect(gate.shouldOverride("session-2")).toBe(false)
+  })
+})