feat: unified git repo hook + fix branch dropdown in workspace creation

- New useGitRepo hook: centralizes all git operations (branches, status,
  history, commit, push, pull, fetch, checkout, create/delete branch, merge)
  for a given project+repo. Auto-refreshes after mutating ops.
- Fix WorkspaceCreateForm branch dropdown:
  - Always renders <select> (never input fallback)
  - Uses useGitRepo for branch fetching with loading/error states
  - Shows 'Loading branches...' while fetching
  - Shows 'Enter branch name manually...' if API fails
  - '+ Create new branch...' option with text input reveal
  - Auto-selects default branch on load
- ProjectsPage uses updated form props (defaultProjectId/defaultRepoId)

Quality gates: tsc --noEmit clean, eslint clean

apps/web/src/pages/projects.tsx

@@ -10,10 +10,7 @@ import {
 	type ProjectCreateInput,
 	type ProjectUpdateInput,
 } from "../api/projects";
-import {
-	deleteWorkspace,
-	syncWorkspace,
-} from "../api/workspaces";
+import { deleteWorkspace, syncWorkspace } from "../api/workspaces";
 import {
 	EmptyState,
 	ErrorState,
@@ -111,8 +108,6 @@ export const ProjectsPage = () => {
 		}
 	};
 
-
-
 	const handleSyncWorkspace = async (
 		projectId: string,
 		repoId: string,