fix(git-gate): make the gateway access-hook executable regardless of copy transport

Cloning/fetching from the git-gate on the Apple-container backend failed with
"empty reply from server" (curl exit 52). Root cause: the git-http handler
crashed on every upload-pack with

    PermissionError: [Errno 13] Permission denied: '/etc/git-gate/access-hook'

The access-hook is exec'd directly, so it needs the x bit. prepare() stages it
0o700 and trusted the gateway copy to carry that mode. `docker cp` does; the
Apple `container cp` (AppleGatewayTransport) does not, landing the hook 0o644 →
EACCES. The unhandled exception killed the handler thread, closing the socket
with no HTTP response — which the client sees as the opaque empty reply.

- provision_git_gate now `chmod +x`es the access-hook on the gateway side after
  the copy, so it's executable under every transport (docker/apple/firecracker).
- git-http handler wraps the access-hook subprocess.run: an OSError /
  SubprocessError (un-execable, timed out) now fails closed with a 503 instead
  of crashing the thread into an empty reply — a gate that can't run its hook
  should deny, visibly.
- Updates the now-misleading "docker cp preserves source mode" comment in
  git_gate.prepare().

Regression tests: provisioning applies +x to the access-hook; the handler
returns 503 (not an empty reply) when the hook can't be exec'd.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_gateway_provision.py

@@ -69,6 +69,18 @@ class TestProvisionGitGate(unittest.TestCase):
         self.assertEqual(1, len(exec_scripts))
         self.assertIn("repo=/git/bottle1/${name}.git", exec_scripts[0][-1])
 
+    def test_makes_access_hook_executable_on_the_gateway(self) -> None:
+        # Regression: the access-hook is exec'd directly, so it needs the x
+        # bit. The copy alone can't be trusted to carry the staged 0o700
+        # (`docker cp` preserves mode, the Apple `container cp` does not),
+        # so provisioning must re-apply +x on the gateway side.
+        calls: list[list[str]] = []
+        with patch(_RUN, side_effect=_recorder(calls)):
+            provision_git_gate(DockerGatewayTransport("gw"), "bottle1", _plan(_up("foo")))
+        self.assertIn(
+            ["docker", "exec", "gw", "chmod", "+x", "/etc/git-gate/access-hook"], calls,
+        )
+
     def test_omits_known_hosts_copy_when_absent(self) -> None:
         calls: list[list[str]] = []
         with patch(_RUN, side_effect=_recorder(calls)):