feat(palette): move the scheme selection out of the repo

The selector was tracked at palette/roles.conf, which made switching scheme a
committable change: the previous commit carried `scheme = tokyo-night` in its
diff purely because that was the theme running at the time.

Which theme a machine runs is local state, not a property of the project, so
the live file is now ~/.config/udt/roles.conf. install.sh seeds it from
palette/roles.conf.default on a first run and never overwrites it, so a
switch leaves the working tree clean and two machines sharing this repo can
run different schemes. udt-palette falls back to the seed when no live file
exists, so a bare checkout still generates.

This is the one exception to the repo holding the canonical copy of
everything, and AGENTS.md says so.

Verified: seeding on a missing file, the seed not overwriting an existing
choice, generation from the seed alone, and both selftests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015gbjA2bmswN8jyPDKzrvqe

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