Add the illustrated user guide

A LaTeX field guide covering install, keys, everyday commands, the
decoy passphrase, scripting, locking and sync, internals, troubleshooting
and a cheat sheet. build-pdf.sh compiles it with a pinned Tectonic,
downloaded on first use. shots.py regenerates every screenshot from the
real vault binary in a sandbox home.

.gitignore

@@ -2,3 +2,7 @@
 /vault.exe
 .claude/
 /dist
+/docs/guide/.tools/
+/docs/guide/version.tex
+/docs/guide/vault-guide.pdf
+/docs/guide/*.log