fix(config): normalize configured default_agent

Agent keys are remapped to display names, so preserving `default_agent`
values could still select a missing key at runtime.

This regression surfaced after d94a739203 remapped `config.agent` keys
to display names without canonicalizing configured defaults.

Normalize configured `default_agent` through display-name mapping before
fallback logic and extend tests to cover canonical and display-name
inputs.

src/plugin-handlers/config-handler.test.ts

@@ -350,7 +350,7 @@ describe("Agent permission defaults", () => {
 })
 
 describe("default_agent behavior with Sisyphus orchestration", () => {
-  test("preserves existing default_agent when already set", async () => {
+  test("canonicalizes configured default_agent key to display name", async () => {
     // #given
     const pluginConfig: OhMyOpenCodeConfig = {}
     const config: Record<string, unknown> = {
@@ -371,7 +371,32 @@ describe("default_agent behavior with Sisyphus orchestration", () => {
     await handler(config)
 
     // #then
-    expect(config.default_agent).toBe("hephaestus")
+    expect(config.default_agent).toBe(getAgentDisplayName("hephaestus"))
+  })
+
+  test("preserves existing display-name default_agent", async () => {
+    // #given
+    const pluginConfig: OhMyOpenCodeConfig = {}
+    const displayName = getAgentDisplayName("hephaestus")
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      default_agent: displayName,
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    // #when
+    await handler(config)
+
+    // #then
+    expect(config.default_agent).toBe(displayName)
   })
 
   test("sets default_agent to sisyphus when missing", async () => {