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>

README.md

@@ -41,10 +41,13 @@ New to git? Start here: https://git-scm.com/doc
 `<Module>` below is one of the top-level directories of the cloned repo
 (e.g. `/das/work/p21/p21515/MX_Pmodule/ccp4`):
 
+**WARNING: never write big files to `/var/tmp` — it is small and shared with
+everyone on the login node. Please, please do not fill /var/tmp.**
+
 Always pass `--tmpdir` and `--distdir` pointing to your p-group work folder —
-the defaults are `/var/tmp/$USER` (shared with everyone on the login node —
-**do NOT fill it**) and `~/.cache/Pmodules/distfiles` (home quota is too
-small). Adapt the p-group path (`p21/p21515`) to your own:
+the defaults are `/var/tmp/$USER` (see warning above) and
+`~/.cache/Pmodules/distfiles` (home quota is too small). Adapt the p-group
+path (`p21/p21515`) to your own:
 
 ```bash
 module use unstable && module load modbuild/2.1.3
@@ -100,6 +103,13 @@ install-type archetype and prints the Ra build recipe.
 - **Other agents** (Cursor, Codex, generic LLM chat): the skill is plain
   Markdown — point the agent at `.claude/skills/new-module/SKILL.md` first,
   then `REFERENCE.md`, and ask it to follow them.
+- **New software (or a known tool that moved to a new download site)**: give
+  the agent the software's installation/download page as part of the context —
+  the skill knows the repo's patterns, not vendor-specific install procedures.
+- **The skill can be buggy** — use it with caution and double-check what it
+  scaffolds before building; corrections are welcome (edit
+  `.claude/skills/new-module/` and commit). Reading along while it works is
+  also a good way to learn how these modules are put together.
 - The skill copies from real modules in this repo as ground truth (`xds`,
   `careless`, `DIALS`, `ccp4`, ...) — keep those exemplary, and fold new
   lessons back into the skill files so the next build starts smarter.