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.

CHANGELOG.md

@@ -84,15 +84,24 @@ base** rather than reusing `base-6f3b20266de1`.
     (unless the document sets `header-includes`). Under typst, pandoc exits
     0 and prints that header as text above the first heading. Seen in the same PDF through two renderers, MuPDF and
     Chromium.
-  - **`texlive-xetex` alone, which the README offered as the fallback, is
-    not enough.** The export then still lacks `lmodern.sty`, `ulem.sty`
-    and the `pzdr` font. Adding `lmodern texlive-plain-generic
-    texlive-fonts-recommended` makes it work: 37 packages and 491 MB
-    installed, after `apt-get update`. Measured on v1.11.2 with pi-studio
-    0.10.1. The exported page was looked at, not only its exit code.
+  - **`texlive-xetex`, the README's old fallback, works but is twice the
+    size it needs to be.** Installed with apt's defaults it pulls in its
+    recommends: 202 packages and 1053 MB, including systemd, D-Bus and a
+    Java runtime (`openjdk-21-jre-headless`, 194 MB). The same 202 come from
+    the old `AGENTS.md` recipe, `texlive-xetex texlive-latex-recommended`.
+    With `--no-install-recommends`, `texlive-xetex` alone is not enough:
+    the export then lacks `lmodern.sty`, `ulem.sty` and the `pzdr` font.
+    The README's recipe adds the three packages that provide them,
+    `lmodern texlive-plain-generic texlive-fonts-recommended`, and comes to
+    37 packages and 491 MB, after `apt-get update`. Each recipe was
+    measured from the image's own 401 packages on v1.11.2 with pi-studio
+    0.10.1, and the exported page was looked at, not only its exit code.
 
-  The same claim also appears in `AGENTS.md` ("What we DON'T install") and
-  in comments in `Dockerfile.base`. Neither is changed here.
+  The same claim is corrected in `AGENTS.md` ("What we DON'T install"),
+  which now also warns against `ENV PANDOC_PDF_ENGINE=typst`, and in two
+  comments in `Dockerfile.base`'s typst block. `base_tag` hashes all of
+  `Dockerfile.base`, comments included, so this changes the base hash too.
+  The base is rebuilt in the next release anyway (above).
 
 ## v1.11.2 — 2026-10-14