feat: bundle dot-watch helper for live graphviz .dot -> PNG re-render in Studio

pi-studio renders Mermaid natively but has no DOT renderer. Its markdown
preview displays local PNG/JPG/GIF/WEBP images, so dot-watch closes the
loop for Graphviz: edit .dot -> auto-render <name>.png -> Studio
refresh-from-disk shows the update. Uses mtime polling (no inotify dep).

- rootfs/usr/local/bin/dot-watch: the helper (executable)
- Dockerfile.base: COPY + chmod, following the studio-expose pattern
- README.md: 'Graphviz diagrams in Studio' subsection
- CHANGELOG.md: Unreleased entry

graphviz was already in the base image; no new package.

CHANGELOG.md

@@ -13,6 +13,18 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
 
 ## Unreleased
 
+### Added
+
+- **`dot-watch` helper** (`/usr/local/bin/dot-watch`) — auto-rerenders a
+  Graphviz `.dot` file to PNG on every save via mtime polling (no
+  `inotify` dependency). pi-studio renders Mermaid natively but has no
+  DOT renderer; since its markdown preview displays local PNG/JPG/GIF/WEBP
+  images, this closes the loop for Graphviz: edit `.dot` → `dot-watch`
+  regenerates `<name>.png` → Studio *refresh-from-disk* shows the update.
+  `graphviz` was already in the base image, so no new package. Baked into
+  `Dockerfile.base` following the `studio-expose` pattern; documented in
+  the README Studio section.
+
 ## v1.1.0 — 2026-06-10
 
 ### Added — `:latest-studio` variant