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`).

README.md

@@ -149,8 +149,12 @@ Currently published:
 
 Planned for an upcoming minor release:
 
-- `joakimp/pi-devbox:latest-studio-tex` — `-studio` plus `texlive-xetex`
-  for PDF export from Studio. Adds ~600 MB on top of `-studio`.
+- **PDF export from Studio/pandoc** (a future minor release). Engine still to be decided.
+  A lightweight engine is preferred over a full TeX install:
+  - `typst` — single static binary (~30 MB), `pandoc --pdf-engine=typst`; light
+    enough it could ship in the base image (no separate `-tex` variant needed).
+  - `joakimp/pi-devbox:latest-studio-tex` — `-studio` plus `texlive-xetex`
+    (~600 MB on top of `-studio`); highest fidelity, kept as the fallback.
 
 ## Using pi-studio (`-studio` variant)
 
@@ -285,9 +289,11 @@ Assuming the compose file publishes `127.0.0.1:8765:8765` (see method B):
 > until step 2 runs. If the browser can't connect, verify Studio is up
 > (`/studio --status`) and the bridge is running (`ps aux | grep socat`).
 
-> PDF export (`/studio-pdf`, `studio_export_pdf`) needs a LaTeX engine,
-> which is **not** in `-studio` (only the planned `-studio-tex`). HTML
-> export, KaTeX, Mermaid, and all REPL features work without it.
+> PDF export (`/studio-pdf`, `studio_export_pdf`) needs a PDF engine, which is
+> **not** in `-studio` (pandoc is present but has no engine, so export fails
+> with `xelatex not found`). Planned for a future release — likely a lightweight
+> engine such as `typst`, with `texlive-xetex` as the heavier fallback. HTML export,
+> KaTeX, Mermaid, and all REPL features work without it.
 
 ### Graphviz diagrams in Studio: `dot-watch`