feat(proofs): add Agda project file for proofs/agda/

Adds echidna-proofs.agda-lib so Agda resolves stdlib imports when
invoked from the proofs/agda/ directory. ProofComposition.agda
type-checks clean under Agda 2.8.0 and ECHIDNA returns ✓ valid.

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

Author: hyperpolymath

proofs/agda/echidna-proofs.agda-lib

@@ -0,0 +1,5 @@
+-- SPDX-License-Identifier: PMPL-1.0-or-later
+-- Agda library file for ECHIDNA dogfood proofs
+name: echidna-proofs
+depend: standard-library
+include: .