refactoring: switched to gpt-5-nano for commit message generation

Remove commitMessageModel from persisted settings and defaults
Eliminate UI controls and state for commitMessageModel in GitSettings and config store
Update backend and persistence layers to stop reading or writing commitMessageModel

packages/ui/src/lib/desktop.ts

@@ -59,7 +59,6 @@ export type DesktopSettings = {
   defaultGitIdentityId?: string; // ''/undefined = unset, 'global' or profile id
   autoCreateWorktree?: boolean;
   queueModeEnabled?: boolean;
-  commitMessageModel?: string; // format: "provider/model"
   gitmojiEnabled?: boolean;
 
   // User-added skills catalogs (persisted to ~/.config/openchamber/settings.json)

packages/ui/src/lib/persistence.ts

@@ -47,11 +47,6 @@ const persistToLocalStorage = (settings: DesktopSettings) => {
   } else {
     localStorage.removeItem('pinnedDirectories');
   }
-  if (settings.commitMessageModel) {
-    localStorage.setItem('commitMessageModel', settings.commitMessageModel);
-  } else {
-    localStorage.removeItem('commitMessageModel');
-  }
   if (typeof settings.gitmojiEnabled === 'boolean') {
     localStorage.setItem('gitmojiEnabled', String(settings.gitmojiEnabled));
   } else {
@@ -271,9 +266,6 @@ const sanitizeWebSettings = (payload: unknown): DesktopSettings | null => {
   if (typeof candidate.autoCreateWorktree === 'boolean') {
     result.autoCreateWorktree = candidate.autoCreateWorktree;
   }
-  if (typeof candidate.commitMessageModel === 'string' && candidate.commitMessageModel.length > 0) {
-    result.commitMessageModel = candidate.commitMessageModel;
-  }
   if (typeof candidate.gitmojiEnabled === 'boolean') {
     result.gitmojiEnabled = candidate.gitmojiEnabled;
   }