fix(create_project): make the write the branch-readiness gate (#42)

BranchExists returns true before the generated branch is writable, so
waitForBranch didn't help and a 2.5s retry budget was too short (branch became
writable ~30s post-generate under load in live testing). Drop waitForBranch;
let upsertRetry be the gate — retry the write on the transient "branch does not
exist" not-found for up to 60s, early-exit on success. Once the first write
lands the branch is writable and the rest succeed immediately. Refs #42.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/tools/create_project_from_template_test.go

@@ -69,10 +69,6 @@ func (f *fakeTemplateServer) handler(t *testing.T, tmpl, dest string) http.Handl
 			_, _ = fmt.Fprintf(w, `{"name":%q,"full_name":"mathias/%s","default_branch":%q,"clone_url":"http://gitea.example.com/mathias/%s.git","html_url":"http://gitea.example.com/mathias/%s","template":false}`,
 				dest, dest, f.genBranch, dest, dest)
 
-		case r.Method == http.MethodGet && strings.HasPrefix(p, "/api/v1/repos/mathias/"+dest+"/branches/"):
-			// waitForBranch readiness probe — branch is present.
-			_, _ = fmt.Fprint(w, `{"name":"main","commit":{"id":"c"}}`)
-
 		case r.Method == http.MethodGet && strings.HasPrefix(p, "/api/v1/repos/mathias/"+dest+"/git/trees/"):
 			var entries []string
 			for path := range f.files {