docs: thin SPEC.adoc §8 to forward-reference codegen-environment.adoc

PR #94 landed the full codegen environment reference as
docs/specs/codegen-environment.adoc. §8 in SPEC.adoc is now redundant;
replace its body with a one-paragraph forward-reference to avoid
maintaining two copies.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath