fix: respect default_agent in sort shim ordering (#3900)

The sort shim always placed core agents (sisyphus, hephaestus, prometheus,
atlas) before any custom agent, ignoring the user's configured default_agent
in opencode.json. This caused the TUI to always show Sisyphus as the selected
agent on startup regardless of default_agent.

Add setDefaultAgentForSort() which inserts the configured default_agent at
rank 0 in the sort shim's rank map. Called from applyAgentConfig after
resolving the effective default_agent value.

Fixes #3900

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

@@ -8,6 +8,7 @@ import {
   normalizeAgentForPromptKey,
 } from "../shared/agent-display-names";
 import { AGENT_NAME_MAP } from "../shared/migration";
+import { setDefaultAgentForSort } from "../shared/agent-sort-shim";
 import { registerAgentName } from "../features/claude-code-session-state";
 import {
   discoverConfigSourceSkills,
@@ -399,6 +400,11 @@ export async function applyAgentConfig(params: {
     );
   }
 
+  const resolvedDefault = (params.config as { default_agent?: string }).default_agent;
+  if (resolvedDefault) {
+    setDefaultAgentForSort(resolvedDefault);
+  }
+
   const agentResult = params.config.agent as Record<string, unknown>;
   for (const name of Object.keys(agentResult)) {
     registerAgentName(name);