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>

bot_bottle/backend/docker/gateway_provision.py

@@ -94,6 +94,12 @@ def provision_git_gate(
     transport.exec(["mkdir", "-p", "/etc/git-gate"])
     transport.cp_into(str(plan.hook_script), "/etc/git-gate/pre-receive")
     transport.cp_into(str(plan.access_hook_script), "/etc/git-gate/access-hook")
+    # The access-hook is exec'd directly (not via `sh`), so it needs the x bit.
+    # Set it here rather than trusting the copy to carry the staged 0o700:
+    # `docker cp` preserves source mode, but the Apple `container cp` does not,
+    # landing the hook 0o644 → EACCES when the git-http handler tries to exec it.
+    # chmod on the gateway side is backend-neutral and fixes every transport.
+    transport.exec(["chmod", "+x", "/etc/git-gate/access-hook"])
     creds = _creds_dir(bottle_id)
     transport.exec(["mkdir", "-p", creds])
     for u in plan.upstreams: