fix: resolve ProviderModelNotFoundError on non-Claude runtimes (#1156)

Extend resolve_model_ids to accept "omit" value: returns empty string
so non-Claude runtimes (OpenCode, Codex, Gemini, etc.) use their
configured default model instead of unresolvable Claude aliases.

- resolve_model_ids: "omit" short-circuits before alias resolution
- model_overrides still respected (checked first) for explicit IDs
- Installer sets resolve_model_ids: "omit" in ~/.gsd/defaults.json
  for non-Claude runtimes during install
- 4 new tests covering omit behavior and override passthrough
- Fix websearch test mocks for fs.writeSync output change (#1276)

Closes #1156

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/install.js

@@ -4171,6 +4171,27 @@ function finishInstall(settingsPath, settings, statuslineCommand, shouldInstallS
     configureOpencodePermissions(isGlobal);
   }
 
+  // For non-Claude runtimes, set resolve_model_ids: "omit" in ~/.gsd/defaults.json
+  // so resolveModelInternal() returns '' instead of Claude aliases (opus/sonnet/haiku)
+  // that the runtime can't resolve. Users can still use model_overrides for explicit IDs.
+  // See #1156.
+  if (runtime !== 'claude') {
+    const gsdDir = path.join(os.homedir(), '.gsd');
+    const defaultsPath = path.join(gsdDir, 'defaults.json');
+    try {
+      fs.mkdirSync(gsdDir, { recursive: true });
+      let defaults = {};
+      try { defaults = JSON.parse(fs.readFileSync(defaultsPath, 'utf8')); } catch { /* new file */ }
+      if (defaults.resolve_model_ids !== 'omit') {
+        defaults.resolve_model_ids = 'omit';
+        fs.writeFileSync(defaultsPath, JSON.stringify(defaults, null, 2) + '\n');
+        console.log(`  ${green}✓${reset} Set resolve_model_ids: "omit" in ~/.gsd/defaults.json`);
+      }
+    } catch (e) {
+      console.log(`  ${yellow}⚠${reset} Could not write ~/.gsd/defaults.json: ${e.message}`);
+    }
+  }
+
   let program = 'Claude Code';
   if (runtime === 'opencode') program = 'OpenCode';
   if (runtime === 'gemini') program = 'Gemini';