feat(base): ship typst as lightweight pandoc PDF engine

pandoc has been in the base since v1.0.0 but only as a front-end; PDF
export (studio_export_pdf / pandoc -o out.pdf) failed with 'xelatex not
found' because no back-end engine was installed. Ship typst (~30 MB
static Rust binary, no LaTeX) as the default engine via
`pandoc --pdf-engine=typst`, chosen over a ~600 MB TeX Live install.
texlive-xetex remains the higher-fidelity install-on-demand fallback.

- Dockerfile.base: install typst (latest GitHub-release idiom, pin via
  TYPST_VERSION); add xz-utils (typst ships .tar.xz); bump
  BASE_REBUILD_DATE. Lands in base-<hash>.
- smoke-test.sh: verify typst + a real pandoc --pdf-engine=typst render.
- README/AGENTS/CHANGELOG: typst now shipped (supersedes the planned
  :latest-studio-tex variant).

No tag pushed — CI build intentionally deferred.

AGENTS.md

@@ -153,14 +153,14 @@ deprecated artifacts (to be removed in opencode-devbox v2.0.0).
 
 ## What we DON'T install (and why)
 
-- **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`. 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.
+- **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`).
 - **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`).