openthread-br: Decouple LuCI package from this repository

[BKPepe]

While reviewing #29784, I found, we are shipping not only luci-app-openthread, but also openthread-br.

In ideal world, LuCI app package should be moved to the LuCi repository, where it is going to be reviewed while adding there, when it was not reviewed in initial PR, when the package was added here and most likely, it was not reviewed in downstream repository, where we can find these files - openthread/ot-br-posix#3431.

[LorbusChris]

Migration is in progress:

• luci-app-openthread: add new application luci#8871 — proposes adding luci-app-openthread to the LuCI repo, rewritten for the client-side JS + rpcd ucode framework.
• openthread-br: update to 2026.08.0, internal mDNS, configurable REST listen address #30091 — proposes removing the luci-app-openthread subpackage from net/openthread-br here so the app isn't shipped from two places.

Refs openthread/ot-br-posix#3431 and openthread/ot-br-posix#3326 (removes the old in-tree app).

(Edited: an earlier version of this note said the luci PR depended on a ubus field-name normalization in ot-br-posix; that was dropped — the app reads the daemon's existing fields, so it needs no ot-br-posix change. See the update below.)

[LorbusChris]

Update: openwrt/luci#8871 is now ready for review. It reads otbr-agent's existing ubus fields, so it needs no ot-br-posix change and works against the current package.

#30091 (drop luci-app-openthread from this feed) should land together with openwrt/luci#8871 so the two feeds don't ship a same-named package.