fix(ci): use knitr::rmarkdown vignette engine + ignore gitea/CLAUDE

R CMD build (run by rcmdcheck before R CMD check) rebuilds vignettes
from source regardless of --no-vignettes. Our vignette declared
%\VignetteEngine{knitr::knitr} which requires the 'markdown' package
that isn't in CI's dependency tree. Switch to %\VignetteEngine{knitr::rmarkdown},
which uses the already-Suggests-listed 'rmarkdown' package and matches
the output: rmarkdown::html_vignette directive in the YAML header.

Also add ^\.gitea$ and ^CLAUDE\.md$ to .Rbuildignore so R CMD check
stops emitting the "hidden file" / "non-standard top-level file" notes.

Local rcmdcheck (mirroring CI's exact args) now reports
0 errors / 0 warnings / 0 notes.

The pdflatex notice in the build log is unrelated — R CMD build prints
"Not building PDF manual" and continues; with --no-manual it's silenced
entirely.

vignettes/population-denominators.Rmd

@@ -3,7 +3,7 @@ title: "Population denominators"
 output: rmarkdown::html_vignette
 vignette: >
   %\VignetteIndexEntry{Population denominators}
-  %\VignetteEngine{knitr::knitr}
+  %\VignetteEngine{knitr::rmarkdown}
   %\VignetteEncoding{UTF-8}
 ---