fix(#2852): forward model overrides from categories/agent config to subagents

- fix(switcher): use lastIndexOf for multi-slash model IDs (e.g. aws/anthropic/claude-sonnet-4)
- fix(model-resolution): same lastIndexOf fix in doctor parseProviderModel
- fix(call-omo-agent): resolve model from agent config and forward to both
  background and sync executors via DelegatedModelConfig
- fix(subagent-resolver): inherit category model/variant when agent uses
  category reference without explicit model override
- test: add model override forwarding tests for call-omo-agent
- test: add multi-slash model ID test for switcher

src/hooks/think-mode/switcher.test.ts

@@ -146,6 +146,19 @@ describe("think-mode switcher", () => {
         expect(getHighVariant("custom-llm/gemini-3.1-pro")).toBe("custom-llm/gemini-3-1-pro-high")
       })
 
+      it("should handle multi-slash model IDs (#2852)", () => {
+        // given model IDs with multiple slashes (e.g. aws/anthropic/claude-sonnet-4)
+        const variant = getHighVariant("aws/anthropic/claude-sonnet-4-6")
+
+        // then should split at last slash, preserving full provider prefix
+        expect(variant).toBe("aws/anthropic/claude-sonnet-4-6-high")
+      })
+
+      it("should return null for multi-slash unknown models", () => {
+        // given multi-slash model ID without high variant mapping
+        expect(getHighVariant("aws/anthropic/unknown-model")).toBeNull()
+      })
+
       it("should return null for prefixed models without high variant mapping", () => {
         // given prefixed model IDs without high variant mapping
         expect(getHighVariant("vertex_ai/unknown-model")).toBeNull()

src/hooks/think-mode/switcher.ts

@@ -26,9 +26,10 @@ import { normalizeModelID } from "../../shared"
  * extractModelPrefix("vertex_ai/claude-sonnet-4-6") // { prefix: "vertex_ai/", base: "claude-sonnet-4-6" }
  * extractModelPrefix("claude-sonnet-4-6") // { prefix: "", base: "claude-sonnet-4-6" }
  * extractModelPrefix("openai/gpt-5.4") // { prefix: "openai/", base: "gpt-5.4" }
+ * extractModelPrefix("aws/anthropic/claude-sonnet-4") // { prefix: "aws/anthropic/", base: "claude-sonnet-4" }
  */
 function extractModelPrefix(modelID: string): { prefix: string; base: string } {
-  const slashIndex = modelID.indexOf("/")
+  const slashIndex = modelID.lastIndexOf("/")
   if (slashIndex === -1) {
     return { prefix: "", base: modelID }
   }