* test(scripts): cover discord extra auto-detection
The discord mapping in scripts/detect_uv_extras.py worked but was neither
listed in the "currently maps" docstring nor covered by a test, unlike the
buzz mapping that grew up beside it.
* fix(channels): point the Discord missing-dependency hint at the discord extra
discord.py is the only IM transport that is not a core dependency — it ships in
the optional `discord` extra, which detect_uv_extras.py, backend/Dockerfile and
deploy.sh already expand. Its ImportError hint told users to run `uv add
discord.py` instead, which rewrites pyproject.toml/uv.lock in a tree every
restart syncs with `--locked`. Mirror the buzz hint wording and note the extra
in config.example.yaml next to the buzz block.
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Co-authored-by: huyengiang101086-spec <270684046+huyengiang101086-spec@users.noreply.github.com>