fix(ci): drop flaky Agda job — reliable green Coq + Z3 (#105)

Four apt-based Agda setups failed on ubuntu-24.04 (Ubuntu's agda-stdlib
ships no usable library manifest). Rather than ship a perpetually-red
job, this removes Agda from CI — it stays verified by the local gate
`proofs/verify-all-provers.sh`. **Coq (14 theories, both pillars) + Z3
remain, both reliably green.** The workflow documents the reliable
re-add route (pinned Agda toolchain, not apt).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/proofs.yml

@@ -48,27 +48,14 @@ jobs:
           make -f Makefile.all -j"$(nproc)"
           echo "✓ Coq: 14/14 theories compiled (CNO + OND)"
 
-  agda:
-    name: Agda — CNO + OND
-    runs-on: ubuntu-24.04
-    steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v4
-      - name: Install Agda + stdlib
-        run: sudo apt-get update && sudo apt-get install -y agda agda-stdlib
-      - name: Type-check CNO + OND (--safe --without-K)
-        working-directory: proofs/agda
-        run: |
-          agda --version
-          # The Ubuntu agda-stdlib package installs the SOURCES (no .agda-lib
-          # manifest), so add its root directly to Agda's include path via -i.
-          # Locate the root as the parent of Data/Nat.agda (version-dir robust).
-          NAT="$(dpkg -L agda-stdlib | grep -m1 '/Data/Nat\.agda$')"
-          STDLIB="${NAT%/Data/Nat.agda}"
-          echo "stdlib include path: $STDLIB"
-          [ -d "$STDLIB" ] || { echo "::error::agda-stdlib source dir not found"; dpkg -L agda-stdlib | tail -60; exit 1; }
-          agda --safe --without-K -i "$STDLIB" CNO.agda
-          agda --safe --without-K -i "$STDLIB" OND.agda
-          echo "✓ Agda: CNO + OND type-check"
+  # NOTE: an Agda job was attempted but removed. Ubuntu's `agda-stdlib` package
+  # ships the sources without a usable library manifest, and the apt Agda/stdlib
+  # pairing on the runner would not resolve `standard-library` reliably across
+  # four different setups (register / -i include path / synthesized .agda-lib).
+  # Rather than ship a flaky red job, Agda is verified via the local/container
+  # gate `proofs/verify-all-provers.sh` (which type-checks CNO.agda + OND.agda).
+  # To re-add it, a pinned Agda toolchain (e.g. a Nix/setup-agda action with a
+  # matching agda-stdlib) is the reliable route — not apt.
 
   z3:
     name: Z3 — CNO + OND bounded checks