bridge-render.sh snapshot depends on the host's /opt/roadie-config/channels.yaml #776

Problem

tests/bridge-render.sh passes in CI, but it fails on a managed host. The test depends on ambient host state instead of fixtures it creates.

On a host where /opt/roadie-config/channels.yaml exists, the systemd render adds an extra line, so the roadie-systemd snapshot drifts:

+Environment=ROADIE_CHANNELS_CONFIG="/opt/roadie-config/channels.yaml"

Seen on extrachill.com while verifying #773 and #775. It's identical on origin/main, so it's not caused by either PR. The result is that a local run of the test suite can't be trusted as a gate on a real host.

Target state

• The render test isolates the config root, either by pointing it at a temp dir or by stubbing the channels-config existence check.
• The snapshot is deterministic whether or not the host has a real channels.yaml.
• One case asserts that ROADIE_CHANNELS_CONFIG is rendered when the fixture provides the file.

Deliverable

A committed change with tests. bash tests/bridge-render.sh passes on a host with /opt/roadie-config/channels.yaml present and in CI. CHANGELOG and VERSION are release-owned and stay as they are.