ci(agda): install absolute-zero library so the suite typechecks

The echo-types.agda-lib declares `depend: standard-library
absolute-zero`, but the workflow only registered standard-library.
This has been red on main since 3f654ef (2026-04-30, EchoCNOBridge
content-bridge promotion) added the dependency without updating CI.

Adds a fetch step that shallow-clones hyperpolymath/absolute-zero,
then registers it alongside standard-library in ~/.agda/libraries.

Author: hyperpolymath

.github/workflows/agda.yml

@@ -30,7 +30,14 @@ jobs:
           git clone --depth 1 --branch v2.3 https://github.com/agda/agda-stdlib.git "$STDLIB_DIR"
           echo "stdlib_dir=$STDLIB_DIR" >> "$GITHUB_OUTPUT"
 
-      - name: Register standard library for Agda
+      - name: Fetch absolute-zero library
+        id: absz
+        run: |
+          ABSZ_DIR="$RUNNER_TEMP/absolute-zero"
+          git clone --depth 1 https://github.com/hyperpolymath/absolute-zero.git "$ABSZ_DIR"
+          echo "absz_dir=$ABSZ_DIR" >> "$GITHUB_OUTPUT"
+
+      - name: Register libraries for Agda
         run: |
           mkdir -p "$HOME/.agda"
 
@@ -40,7 +47,13 @@ jobs:
           include: ${{ steps.stdlib.outputs.stdlib_dir }}/src
           EOF
 
-          printf '%s\n' "$STDLIB_LIB" > "$HOME/.agda/libraries"
+          ABSZ_LIB="$RUNNER_TEMP/absolute-zero.agda-lib"
+          cat > "$ABSZ_LIB" <<EOF
+          name: absolute-zero
+          include: ${{ steps.absz.outputs.absz_dir }}/proofs/agda
+          EOF
+
+          printf '%s\n%s\n' "$STDLIB_LIB" "$ABSZ_LIB" > "$HOME/.agda/libraries"
           printf '%s\n' "standard-library" > "$HOME/.agda/defaults"
 
       - name: Typecheck full suite