Replace the inherited upstream README with a Hellion-specific one
that lists the privacy/retention/cleanup/export features, links to
upstream and the relevant unanswered filtering issues, documents
the EUPL-1.2 license relationship, and acknowledges Infi & Anna for
the Chat 2 engine that everything builds on.
Add AI_DISCLOSURE.md with the goatcorp Pair classification, an
explicit list of what AI is and is not used for in this fork
(translations, visual assets and license-sensitive boundaries are
handled by the maintainer), and the tooling list. Drops in before
v0.1 so it's already in place when the repo goes public.
HellionStyle.Push() returns a disposable bundle of ImGui color
pushes (cyan-teal accents on a deep-slate frame with steel
borders) and pops them in reverse on Dispose. Privacy tab and the
first-run wizard wrap their Draw with `using var _style =
HellionStyle.Push()` so only Hellion-owned surfaces get the
HUD-flavored palette while upstream Chat 2 tabs render in their
original style — important so cherry-picks from upstream don't
fight with our color overrides.
