ci: run the repo-stats suite and a report-only push step in nix-build (improvise-1xfb.5.1)

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

.github/workflows/ci.yml

@@ -146,13 +146,38 @@ jobs:
   # dependency crate each run. The NixOS runners substitute from titan's
   # harmonia first, and titan builds gh/main's `#improvise` hourly, so a PR that
   # leaves Cargo.lock alone rebuilds only the workspace crates it changed.
+  #
+  # The repo-stats pipeline runs here because it needs this job's binary:
+  # `nix shell .#improvise` reuses the build above instead of a second one.
+  # Its suite gates; the report on a push to main never fails the build, and
+  # its step fails only when improvise refuses the pipeline's input
+  # (context/code/tooling.md section 7). Full history is for the report.
   nix-build:
-    name: Nix build (crate2nix)
+    name: Nix build (crate2nix) and the repo-stats pipeline
     runs-on: [self-hosted, nixos, microvm]
     steps:
       - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
       - name: nix build .#improvise
         run: nix build --print-build-logs .#improvise
+      - name: The repo-stats pipeline computes what it claims to
+        run: nix shell .#improvise --command ./scripts/checkers.sh --ci nix-build
+      - name: Churn, coupling and doc-growth report (report only)
+        if: github.event_name == 'push'
+        continue-on-error: true
+        run: |
+          rc=0
+          nix shell .#improvise --command ./scripts/repo-stats --out repo-stats > repo-stats.txt || rc=$?
+          { echo '```'; cat repo-stats.txt; echo '```'; } >> "$GITHUB_STEP_SUMMARY"
+          exit "$rc"
+      - name: Upload the repo-stats model and its CSVs
+        if: github.event_name == 'push'
+        uses: actions/upload-artifact@v7
+        with:
+          name: repo-stats
+          path: repo-stats
+          retention-days: 30
 
   wasm:
     name: wasm32 check (browser bundle crates)