fix: remove hardcoded model defaults from categories and agents

BREAKING CHANGE: Model resolution overhauled

- Created centralized model-resolver.ts with priority chain:
  userModel → inheritedModel → systemDefaultModel
- Removed model field from all 7 DEFAULT_CATEGORIES entries
- Removed DEFAULT_MODEL constants from 10 agents
- Removed singleton agent exports (use factories instead)
- Made CategoryConfigSchema.model optional
- CLI no longer generates model overrides
- Empty strings treated as unset (uses fallback)

Users must now:
1. Use factory functions (createOracleAgent, etc.) instead of singletons
2. Provide model explicitly or use systemDefaultModel
3. Configure category models explicitly if needed

Fixes model fallback bug where hardcoded defaults overrode
user's OpenCode configured model.

src/shared/model-resolver.test.ts

@@ -0,0 +1,101 @@
+import { describe, expect, test } from "bun:test";
+import { resolveModel, type ModelResolutionInput } from "./model-resolver";
+
+describe("resolveModel", () => {
+  describe("priority chain", () => {
+    test("returns userModel when all three are set", () => {
+      // #given
+      const input: ModelResolutionInput = {
+        userModel: "anthropic/claude-opus-4-5",
+        inheritedModel: "openai/gpt-5.2",
+        systemDefault: "google/gemini-3-pro",
+      };
+
+      // #when
+      const result = resolveModel(input);
+
+      // #then
+      expect(result).toBe("anthropic/claude-opus-4-5");
+    });
+
+    test("returns inheritedModel when userModel is undefined", () => {
+      // #given
+      const input: ModelResolutionInput = {
+        userModel: undefined,
+        inheritedModel: "openai/gpt-5.2",
+        systemDefault: "google/gemini-3-pro",
+      };
+
+      // #when
+      const result = resolveModel(input);
+
+      // #then
+      expect(result).toBe("openai/gpt-5.2");
+    });
+
+    test("returns systemDefault when both userModel and inheritedModel are undefined", () => {
+      // #given
+      const input: ModelResolutionInput = {
+        userModel: undefined,
+        inheritedModel: undefined,
+        systemDefault: "google/gemini-3-pro",
+      };
+
+      // #when
+      const result = resolveModel(input);
+
+      // #then
+      expect(result).toBe("google/gemini-3-pro");
+    });
+  });
+
+  describe("empty string handling", () => {
+    test("treats empty string as unset, uses fallback", () => {
+      // #given
+      const input: ModelResolutionInput = {
+        userModel: "",
+        inheritedModel: "openai/gpt-5.2",
+        systemDefault: "google/gemini-3-pro",
+      };
+
+      // #when
+      const result = resolveModel(input);
+
+      // #then
+      expect(result).toBe("openai/gpt-5.2");
+    });
+
+    test("treats whitespace-only string as unset, uses fallback", () => {
+      // #given
+      const input: ModelResolutionInput = {
+        userModel: "   ",
+        inheritedModel: "",
+        systemDefault: "google/gemini-3-pro",
+      };
+
+      // #when
+      const result = resolveModel(input);
+
+      // #then
+      expect(result).toBe("google/gemini-3-pro");
+    });
+  });
+
+  describe("purity", () => {
+    test("same input returns same output (referential transparency)", () => {
+      // #given
+      const input: ModelResolutionInput = {
+        userModel: "anthropic/claude-opus-4-5",
+        inheritedModel: "openai/gpt-5.2",
+        systemDefault: "google/gemini-3-pro",
+      };
+
+      // #when
+      const result1 = resolveModel(input);
+      const result2 = resolveModel(input);
+
+      // #then
+      expect(result1).toBe(result2);
+    });
+  });
+});