Add pptx slide-deck build target for screen-share recordings

Same markdown now also builds a PowerPoint deck (make slides /
make template.pptx) alongside the existing PDF, for recording
walkthrough videos. Styling comes from reference.pptx since pandoc's
pptx writer ignores style.css; its theme was hand-tuned to match the
PDF's dark palette and fonts.

Restructure the closing block into a two-column div so the author
photo survives in slide exports (pandoc drops images sharing a plain
div with paragraph text), with a CSS override so the PDF keeps its
original centered layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -25,6 +25,31 @@ make clean                      # removes generated PDFs (never touches the .md
 `make` finds the source `.md` file automatically from the `.pdf` name you ask for, so
 building any filled-in template is just naming the PDF you want.
 
+## Building slides
+
+For recording a screen-share walkthrough of the guide, the same markdown builds
+a PowerPoint deck instead of a PDF:
+
+```sh
+make template.pptx              # slide deck for the base version
+make partners/<slug>.pptx        # a specific partnership's deck
+make slides                      # template.pptx + every partners/*.pptx
+```
+
+Pandoc turns each `##` heading into its own slide, matching the "one step per
+page" layout of the PDF. If you want scripted talking points per slide, add a
+`::: notes` ... `:::` fenced div under a heading — pandoc exports those as
+speaker notes rather than slide body text.
+
+Pandoc's pptx writer doesn't read `style.css` (pptx isn't HTML-based), so the
+slide theme instead comes from `reference.pptx` — a PowerPoint file whose
+slide master carries the colors and fonts, built once by hand-editing its
+theme XML to mirror `style.css`'s dark background, heading blue, body text,
+and accent colors, plus Roboto/Open Sans fonts. Edit it in PowerPoint/Keynote/
+Google Slides (or its `ppt/theme/theme1.xml` and
+`ppt/slideMasters/slideMaster1.xml` directly) if the slide theme ever needs to
+diverge from or resync with the PDF's.
+
 ## Starting a new partnership
 
 ```sh
@@ -49,3 +74,10 @@ make partners/<slug>.pdf
   file, just update the front matter.
 - `style.css` is the single source of truth for the visual theme. Edit it once and
   `make all` rebuilds every partner PDF with the change.
+- The closing block's `:::: {.closing .columns}` / `::: {.column} :::` structure
+  matters for slides: pandoc's pptx writer silently drops an image if it shares a
+  plain div with paragraph text, but keeps it in a two-column div. `.closing .column`
+  in `style.css` flattens those columns back to a centered stack for the PDF, so the
+  printed page looks the same as before. Keep that structure when editing the
+  closing copy — a plain `::: {.closing}` div will make the author photo disappear
+  from the slide deck.