docs(template): add BENCH-RULES.md for engine-building runs

Rules for the runs were being made up mid-run (git for rollback, access
to the real-part drawing archive), so each model started under a
different rule set. BENCH-RULES.md writes them down in one place, stamped
into every new engine, so all models work under the same rules:
workspace limits and no searching for other engines, git init plus
commit-per-working-state, the archive as read-only with nothing
customer-identifying kept in the (publishable) engine folder, tests may
only be added to, and what the final report must cover.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

_Template/README.md

@@ -11,6 +11,9 @@ several of them and keep the best result.
 The decisions that make it an engine must be yours: which sheet(s) to use, which parts go
 where and in what order, which pattern/strategy to apply to which region, and when to stop.
 
+**Read `BENCH-RULES.md` before starting.** It covers your workspace, version control, the
+real-part drawing archive, tests and your final report.
+
 ## Allowed building blocks
 
 Reuse is encouraged. These are tools you drive, composed by your own decision logic: