docs: note lightweight PDF engine (typst) as preferred over full texlive

PDF export from Studio/pandoc still isn't shipped. Record the engine decision
in the living docs (README + AGENTS): pandoc is in the image but has no PDF
back-end, so export fails with 'xelatex not found'. Prefer a lightweight engine
— typst (~30 MB static binary, 'pandoc --pdf-engine=typst') which is small
enough it could ship in base rather than needing a separate ':latest-studio-tex'
variant; texlive-xetex (~600 MB) kept as the higher-fidelity fallback. Also drop
stale 'v1.3.0' pins (v1.3.0 already shipped without PDF) in favour of 'a future
release'. CHANGELOG history left untouched.

AGENTS.md

@@ -155,8 +155,12 @@ deprecated artifacts (to be removed in opencode-devbox v2.0.0).
 
 - **No texlive** (~600 MB–1 GB). Users who need PDF export from pandoc
   or pi-studio can install on demand: `sudo apt-get install texlive-xetex
-  texlive-latex-recommended`. The planned `:latest-studio-tex` variant
-  will bake this in.
+  texlive-latex-recommended`. PDF export is planned for a future release — but
+  note pandoc alone can't render PDF (it needs a back-end engine), and a full TeX
+  install is heavy. A lightweight engine like **`typst`** (~30 MB static
+  binary, `pandoc --pdf-engine=typst`) is the preferred candidate and, being
+  small, could land in base rather than a dedicated `:latest-studio-tex`
+  variant. `texlive-xetex` stays the higher-fidelity fallback.
 - **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`).