Sync new-module skill with current practice; README agent caveats

Skill (local + repo copy): --tmpdir/--distdir now part of the standard
recipe (never write big files to /var/tmp — small and shared on the
login node), env-contamination and slow-shared-storage guardrails
added, ccp4 archetype 5b updated to the wget --continue + gzip -t prep
that is actually in ccp4/build.

Main README: agent section notes the skill can be buggy (use with
caution, corrections welcome, reading along is a good way to learn) and
that new software or a new download site needs the vendor install page
given as context; /var/tmp warning made explicit here and in
ccp4/README.

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

ccp4/README.md

@@ -18,9 +18,9 @@ macromolecular crystallography. https://www.ccp4.ac.uk/
 ## Build on Ra
 
 Build tmp and download cache MUST be on /das, not the defaults
-(/var/tmp is too small, the tarball alone blows the $HOME quota — a
-download dying with "Failure writing output to destination" means
-exactly that). Pass both dirs explicitly on the command line so there
+(/var/tmp is small and **shared with everyone on the login node — do
+not fill it**; the tarball alone blows the $HOME quota — a download
+dying with "Failure writing output to destination" means exactly that). Pass both dirs explicitly on the command line so there
 is no doubt where things land — **adapt the p-group path
 (`p21/p21515`) to your own**: