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/config/schema/agent-overrides.ts

@@ -24,6 +24,8 @@ export const AgentOverrideConfigSchema = z.object({
     .string()
     .regex(/^#[0-9A-Fa-f]{6}$/)
     .optional(),
+  /** Localized display name shown in TUI agent selector (i18n support). Falls back to hardcoded English when not set. */
+  displayName: z.string().optional(),
   permission: AgentPermissionSchema.optional(),
   /** Maximum tokens for response. Passed directly to OpenCode SDK. */
   maxTokens: z.number().optional(),