fix(projects): worktree-create sent repoPath but the server reads path (400)

The 'New worktree' form POSTed { repoPath, branch } to /projects/worktree, but
server.ts:706 and the documented contract (FEATURE_WALKAWAY_WORKBENCH FR-B3.1)
read body.path — so every browser worktree-create returned 400 'path and branch
are required' and git worktree add never ran. Send 'path' instead. The existing
worktree-form test had locked in the wrong key (asserted repoPath), which is why
it stayed green while the feature was broken; corrected to assert 'path'.

test/worktree-form.test.ts

@@ -230,7 +230,7 @@ describe('renderNewWorktreeForm', () => {
     expect(errorEl.style.display).not.toBe('none')
   })
 
-  it('POSTs to /projects/worktree with repoPath and branch on valid input', async () => {
+  it('POSTs to /projects/worktree with path and branch on valid input', async () => {
     const mockFetch = vi.fn().mockResolvedValue({
       ok: true,
       json: async () => ({ ok: true, path: '/home/user/my-repo-worktrees/feat', branch: 'feat' }),
@@ -249,7 +249,9 @@ describe('renderNewWorktreeForm', () => {
       '/projects/worktree',
       expect.objectContaining({
         method: 'POST',
-        body: JSON.stringify({ repoPath: '/home/user/my-repo', branch: 'feat' }),
+        // regression: the server (server.ts) + the docs contract read body.path,
+        // NOT body.repoPath — sending repoPath 400'd every browser worktree-create.
+        body: JSON.stringify({ path: '/home/user/my-repo', branch: 'feat' }),
       }),
     )
   })