fix: unify agent display names and strip invisible sort prefixes

- Replace getAgentRuntimeName with getAgentDisplayName for consistency
- Add stripAgentListSortPrefix helper to normalize agent names
- Strip sort prefixes in subagent-resolver and sync-executor
- Backfill canonical names for core agents when builtin configs omit name
- Update tests to match new behavior

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

@@ -201,7 +201,10 @@ describe("applyAgentConfig builtin override protection", () => {
     })
 
     // then
-    expect(result[BUILTIN_SISYPHUS_DISPLAY_NAME]).toEqual(builtinSisyphusConfig)
+    expect(result[BUILTIN_SISYPHUS_DISPLAY_NAME]).toEqual({
+      ...builtinSisyphusConfig,
+      name: getAgentDisplayName("sisyphus"),
+    })
   })
 
   test("filters user agents whose key differs from a builtin key only by case", async () => {
@@ -223,7 +226,10 @@ describe("applyAgentConfig builtin override protection", () => {
     })
 
     // then
-    expect(result[BUILTIN_SISYPHUS_DISPLAY_NAME]).toEqual(builtinSisyphusConfig)
+    expect(result[BUILTIN_SISYPHUS_DISPLAY_NAME]).toEqual({
+      ...builtinSisyphusConfig,
+      name: getAgentDisplayName("sisyphus"),
+    })
     expect(result.SiSyPhUs).toBeUndefined()
   })
 
@@ -247,7 +253,10 @@ describe("applyAgentConfig builtin override protection", () => {
     })
 
     // then
-    expect(result[BUILTIN_SISYPHUS_DISPLAY_NAME]).toEqual(builtinSisyphusConfig)
+    expect(result[BUILTIN_SISYPHUS_DISPLAY_NAME]).toEqual({
+      ...builtinSisyphusConfig,
+      name: getAgentDisplayName("sisyphus"),
+    })
   })
 
   describe("#given protected builtin agents use hyphenated names", () => {