fix: model format normalization and explicit config cache bypass

- Add normalizeModelFormat() utility for string/object model handling
- Update subagent-resolver to handle both model formats
- Add explicitUserConfig flag to ModelResolutionResult
- Set explicitUserConfig: true when user model is found in pipeline

This fixes the issue where plugin-provided models fail cache validation
and fall through to random fallback models.

src/shared/model-format-normalizer.test.ts

@@ -0,0 +1,46 @@
+import { describe, it, expect } from "bun:test"
+import { normalizeModelFormat } from "./model-format-normalizer"
+
+describe("normalizeModelFormat", () => {
+  describe("string format input", () => {
+    it("splits provider/model format correctly", () => {
+      const result = normalizeModelFormat("opencode/glm-5-free")
+      expect(result).toEqual({ providerID: "opencode", modelID: "glm-5-free" })
+    })
+
+    it("handles provider with multiple slashes", () => {
+      const result = normalizeModelFormat("anthropic/claude-opus-4-6/max")
+      expect(result).toEqual({ providerID: "anthropic", modelID: "claude-opus-4-6/max" })
+    })
+
+    it("returns undefined for malformed string without separator", () => {
+      const result = normalizeModelFormat("invalid")
+      expect(result).toBeUndefined()
+    })
+
+    it("returns undefined for empty string", () => {
+      const result = normalizeModelFormat("")
+      expect(result).toBeUndefined()
+    })
+  })
+
+  describe("object format input", () => {
+    it("passthroughs object format unchanged", () => {
+      const input = { providerID: "opencode", modelID: "glm-5-free" }
+      const result = normalizeModelFormat(input)
+      expect(result).toEqual(input)
+    })
+  })
+
+  describe("edge cases", () => {
+    it("returns undefined for null", () => {
+      const result = normalizeModelFormat(null)
+      expect(result).toBeUndefined()
+    })
+
+    it("returns undefined for undefined", () => {
+      const result = normalizeModelFormat(undefined)
+      expect(result).toBeUndefined()
+    })
+  })
+})