Add channel-aware updater wiring

[d4rken]

Summary

• Add a small channel helper so dev webconfig installs and updates dev components, while stable continues to use update/main, feed/main, and webconfig/master.
• Persist the installed channel in /airplanes/webconfig/channel.env so copied update helpers keep using the same channel.
• Update installer and background updater paths to pass the correct updater/feed/webconfig branches.
• Add CI with shell syntax, shellcheck for the touched scripts, and channel mapping tests.

Tests

• bash test/update-channel-test.sh
• bash -n install.sh update-webconfig.sh helpers/bg-update.sh helpers/update-channel.sh test/update-channel-test.sh
• shellcheck -S warning install.sh update-webconfig.sh helpers/bg-update.sh helpers/update-channel.sh test/update-channel-test.sh