Problem: highlight groups used hardcoded hex colours and a bespoke
hlexists guard, ignoring the user's colorscheme and preventing
overrides from working naturally.
Solution: replace the guard wrapper with direct nvim_set_hl calls
using default = true and link, so each group falls back to a
semantically appropriate built-in group (Title, DiagnosticHint,
DiagnosticError, Comment, DiagnosticWarn) unless the user has
already defined them.