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.

AGENTS.md

@@ -354,14 +354,24 @@ deprecated artifacts (to be removed in opencode-devbox v2.0.0).
 
 ## What we DON'T install (and why)
 
-- **No texlive** (~600 MB–1 GB). PDF export from pandoc / pi-studio works
-  out of the box via **`typst`** (~30 MB static binary), which the base ships
-  as the pandoc PDF engine (`pandoc --pdf-engine=typst`) — small enough to live
-  in base rather than a dedicated `:latest-studio-tex` variant. We don't bake in
-  a full TeX Live: it's heavy and typst covers the common Markdown→PDF case.
-  Users needing LaTeX-exact output can install the higher-fidelity fallback on
-  demand: `sudo apt-get install texlive-xetex texlive-latex-recommended` (then
-  `pandoc --pdf-engine=xelatex`).
+- **No texlive** (~0.5–1 GB). The base ships **`typst`** (~30 MB static
+  binary) as a PDF engine for pandoc, so
+  `pandoc doc.md --pdf-engine=typst -o doc.pdf` works out of the box. That
+  covers the common Markdown→PDF case, small enough to live in base rather
+  than in a dedicated `:latest-studio-tex` variant. Callers must ask for
+  typst: pandoc's own default engine is `pdflatex`, which is not installed.
+  **pi-studio's PDF export (`/studio-pdf`, `studio_export_pdf`) does not use
+  typst.** It runs pandoc with `$PANDOC_PDF_ENGINE`, defaulting to `xelatex`,
+  so in this image it fails with `xelatex not found`. Do not "fix" that with
+  `ENV PANDOC_PDF_ENGINE=typst`. pi-studio adds its own LaTeX header to the
+  document, and typst prints that header as page text while pandoc exits 0.
+  That ENV only becomes a fix once pi-studio stops sending its LaTeX header
+  to non-LaTeX engines. Until then, users who need Studio's PDF export, or
+  LaTeX output, install TeX Live on demand with the recipe in README
+  "PDF export from Studio needs LaTeX" (37 packages, ~490 MB with
+  `--no-install-recommends`). A plain `sudo apt-get install texlive-xetex`
+  also works, but its recommends make it 202 packages and ~1 GB, including
+  systemd and a Java runtime.
 - **pi-studio** ships in the `:latest-studio` variant (since v1.1.0),
   vendored to `/opt/pi-studio` and registered at container start via
   `pi install /opt/pi-studio` (see Dockerfile.variant `INSTALL_STUDIO`).