feat(agents): support per-agent displayName for i18n (#4004)

Add optional `displayName` field to AgentOverrideConfigSchema (next to
the existing `color` field) so users can specify localized agent names
in oh-my-openagent.json:

  { "agents": { "sisyphus": { "displayName": "总指挥" } } }

When set, the override takes precedence everywhere
AGENT_DISPLAY_NAMES[agentName] is used — TUI agent selector, the
agent list key, and the internal `name` field. When not set, behavior
is identical to before (hardcoded English names from AGENT_DISPLAY_NAMES).

Implementation touches:
- AgentOverrideConfigSchema: adds displayName?: z.string().optional()
- getAgentDisplayName / getAgentListDisplayName: accept optional overrides
  map and check displayName before the hardcoded table
- remapAgentKeysToDisplayNames: forwards overrides map to name resolution
- agent-config-handler: passes pluginConfig.agents as the overrides map

Backward compatible — existing configs without displayName continue to
work unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/plugin-handlers/agent-key-remapper.test.ts

@@ -197,4 +197,54 @@ describe("remapAgentKeysToDisplayNames", () => {
       foo: "bar",
     })
   })
+
+  describe("displayName i18n override (#4004)", () => {
+    it("uses per-agent displayName override when set", () => {
+      // given sisyphus config with a Chinese displayName override
+      const agents = {
+        sisyphus: { prompt: "test", mode: "primary" },
+      }
+      const overrides = {
+        sisyphus: { displayName: "总指挥" },
+      }
+
+      // when remapping with overrides
+      const result = remapAgentKeysToDisplayNames(agents, overrides)
+
+      // then the localized name is used instead of "Sisyphus - Ultraworker"
+      expect(result["总指挥"]).toBeDefined()
+      expect((result["总指挥"] as Record<string, unknown>).name).toBe("总指挥")
+      expect(result["Sisyphus - Ultraworker"]).toBeUndefined()
+    })
+
+    it("falls back to hardcoded English name when displayName is not set", () => {
+      // given sisyphus config without displayName override
+      const agents = {
+        sisyphus: { prompt: "test", mode: "primary" },
+      }
+      const overrides = {
+        sisyphus: { model: "claude-opus-4-7" },
+      }
+
+      // when remapping with overrides that have no displayName
+      const result = remapAgentKeysToDisplayNames(agents, overrides)
+
+      // then the legacy AGENT_DISPLAY_NAMES value is used
+      expect(result["Sisyphus - Ultraworker"]).toBeDefined()
+      expect(result["总指挥"]).toBeUndefined()
+    })
+
+    it("falls back to hardcoded English name when no overrides are passed", () => {
+      // given sisyphus config with no overrides at all
+      const agents = {
+        sisyphus: { prompt: "test", mode: "primary" },
+      }
+
+      // when remapping without overrides
+      const result = remapAgentKeysToDisplayNames(agents)
+
+      // then the legacy AGENT_DISPLAY_NAMES value is used
+      expect(result["Sisyphus - Ultraworker"]).toBeDefined()
+    })
+  })
 })