fixup: stateVersion 26.05, drop ubuntu labels, simplify CI workflow

- stateVersion: 25.11 → 26.05 (match hive-gateway + hive-matrix)
- default labels: drop ubuntu-latest/ubuntu-22.04 aliases — misleading
  since this isn't Ubuntu; hive-ci:host is the correct label
- ci.yml: collapse to single `nix flake check` job — flake checks.* already
  covers formatting (treefmt+rustfmt), cargo test, and clippy

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

.forgejo/workflows/ci.yml

@@ -13,30 +13,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: check
-        run: nix flake check --no-build
-
-  fmt:
-    name: formatting
-    runs-on: [hive-ci]
-    steps:
-      - uses: actions/checkout@v3
-      - name: nix fmt
-        run: nix fmt -- --check .
-      - name: cargo fmt
-        run: cargo fmt --all -- --check
-
-  test:
-    name: cargo test
-    runs-on: [hive-ci]
-    steps:
-      - uses: actions/checkout@v3
-      - name: test
-        run: cargo test --workspace
-
-  clippy:
-    name: cargo clippy
-    runs-on: [hive-ci]
-    steps:
-      - uses: actions/checkout@v3
-      - name: clippy
-        run: cargo clippy --workspace -- -D warnings
+        # Runs all flake checks: formatting (treefmt+rustfmt), cargo test,
+        # cargo clippy, and module evaluation. No --no-build: the checks
+        # derivations are the canonical source of truth.
+        run: nix flake check