docs: Studio's PDF export needs LaTeX, in AGENTS.md and Dockerfile.base too

AGENTS.md ("What we DON'T install") and two comments in Dockerfile.base's
typst block still said pi-studio's PDF export uses typst. Same correction
as 77929b7 for the README. AGENTS.md also warns maintainers not to ship
ENV PANDOC_PDF_ENGINE=typst, since pi-studio's LaTeX header is printed as
page text under typst while pandoc exits 0.

Corrects 77929b7: "texlive-xetex alone is not enough" holds only with
--no-install-recommends. With apt's defaults, texlive-xetex (and the old
AGENTS recipe, texlive-xetex texlive-latex-recommended: the same set) works,
but installs 202 packages / 1053 MB including systemd, D-Bus and a JRE.
The README recipe (4 packages, --no-install-recommends) is 37 / 491 MB.
All three recipes measured from the image's 401 packages on v1.11.2 +
pi-studio 0.10.1, checking the exported page, not only the exit code.

Dockerfile.base changes are comments only, but base_tag hashes the whole
file; the next release rebuilds the base anyway.

README.md

@@ -380,10 +380,13 @@ There are two ways out:
     texlive-xetex lmodern texlive-plain-generic texlive-fonts-recommended
   ```
 
-  `texlive-xetex` on its own is not enough. Studio's export then still lacks
-  `lmodern.sty`, `ulem.sty` and the `pzdr` font, which come from the other
-  three packages. The image starts with an empty package index, so run
-  `apt-get update` first. (Measured on v1.11.2 with pi-studio 0.10.1.)
+  The image starts with an empty package index, so run `apt-get update`
+  first. With `--no-install-recommends`, `texlive-xetex` on its own is not
+  enough: Studio's export then lacks `lmodern.sty`, `ulem.sty` and the `pzdr`
+  font, which the other three packages provide. A plain
+  `sudo apt-get install texlive-xetex` also works, but its recommended
+  packages make it 202 packages and about 1 GB, including systemd, D-Bus and
+  a Java runtime. (Measured on v1.11.2 with pi-studio 0.10.1.)
 
 ### Generating a PDF with pandoc + typst