Add bisect_ppx test coverage infrastructure with CI workflow and test suites

Integrate bisect_ppx for code coverage across the test suite:

- Add bisect_ppx instrumentation to lib/dune and test/dune
- Add bisect_ppx dependency to dune-project, nixtamal.opam, and nix/package/nixtamal.nix
- Create bisect.yml configuration for HTML and text coverage reports
- Add .github/workflows/coverage.yml for CI-based coverage reporting
- Fix flake.nix devShell to include checkInputs for full development environment
- Add coverage checks to flake.nix checks output

New test suites for recently ported features:
- test/test_upgrade.ml: Tests for schema upgrade command (backup, dry-run, version validation)
- test/test_fossil.ml: Tests for Fossil VCS codec and lockfile roundtrips
- test/test_lockfile.ml: Tests for lockfile auto-creation and serialization
- test/test_main.ml: Register all new test suites

Documentation updates:
- AGENTS.md: Add contact info (website, XMPP MUC), note llm/ folder is gitignored
- README.asciidoc: Add website link, mention Fossil VCS, schema versioning, upgrade command
- .gitignore: Add _build/ and _coverage/ directories

Covers testing for previously ported features: schema upgrade, Fossil VCS support,
and lockfile auto-creation.

1 files changed, 35 insertions, 0 deletions

diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
new file mode 100644
index 0000000..ee358f0
--- /dev/null
+++ b/.github/workflows/coverage.yml
@@ -0,0 +1,35 @@
+name: coverage
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  test-with-coverage:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: cachix/install-nix-action@v31
+
+      - name: Run tests with bisect_ppx
+        run: |
+          nix develop --option builders '' --command sh -c 'BISECT_ENABLE=YES dune runtest --instrument-with bisect_ppx --force'
+
+      - name: Generate coverage reports
+        run: |
+          nix develop --option builders '' --command sh -c 'bisect-ppx-report summary --coverage-path _build/default/test > coverage-summary.txt'
+          nix develop --option builders '' --command sh -c 'bisect-ppx-report html --coverage-path _build/default/test --tree -o _coverage'
+
+      - name: Upload coverage artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-report
+          path: |
+            coverage-summary.txt
+            coverage/
+            _coverage/
+          if-no-files-found: warn