docs(humans): ship the MD explainer as a PDF with the images baked in

The HTML references screenshots relatively, so it only renders inside a
checkout -- mailing the file on its own gives broken images. The PDF is one
self-contained file, which is what actually gets sent to Ozlem.

- tools/humans_to_pdf.py: renders .humans/*.html via headless Chrome (or
  Edge), so the output is what the browser shows. HTML stays the source.
- @media print block on the page: the wide tables and the pipeline diagram
  scroll on screen but would be cut off at the paper edge; this reflows
  them and keeps figures, callouts and table rows off page breaks.
  Verified at A4 content width -- 6 pages, both screenshots embedded.
- *.pdf -> LFS. Each regeneration is a whole new ~2 MB blob, and unlike
  png/jpg there are no PDFs tracked raw yet, so there is no history to
  rewrite by adding it now.

Also corrects working-files.md: it claimed .gitattributes does not LFS-track
.blend, which was true when written and is no longer -- .blend/.zprj/.obj/
.npy/.npz have since been added. The note now covers what is actually still
excluded (png/jpg, deliberately) and why that makes QA renders scratch.

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

.agents/rules/working-files.md

@@ -66,7 +66,9 @@ pinned.
 
 ## Committing
 
-Scratch patterns are in `.gitignore` and must never be committed. Note that
-`.gitattributes` LFS-tracks `.glb/.fbx/.mp4/.task` but **not `.blend`** — an
-80 MB master committed today goes into git proper, not LFS. Add `.blend` to
-`.gitattributes` first if masters are ever to be tracked.
+Scratch patterns are in `.gitignore` and must never be committed. `.blend`,
+`.zprj`, `.obj`, `.npy`/`.npz` and the rest of the heavy formats are LFS-tracked
+in `.gitattributes`, so a pinned master can be committed safely — but note that
+`.png`/`.jpg` deliberately are **not** (≈250 are already tracked raw; adding them
+would rewrite every one without shrinking history). A lane's QA renders are
+scratch for that reason too, not just a disk one.