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, 6 insertions, 5 deletions

diff --git a/dune-project b/dune-project
index 0ededdd..f023186 100644
--- a/dune-project
+++ b/dune-project
@@ -35,10 +35,11 @@
    (and
     :with-test
     (>= 0.91)))
-  (qcheck-alcotest
-   (and
-    :with-test
-    (>= 0.91))))
- (tags ("nix")))
+   (qcheck-alcotest
+    (and
+     :with-test
+     (>= 0.91)))
+   (bisect_ppx :with-test))
+  (tags ("nix")))
 
 ; See the complete stanza docs at https://dune.readthedocs.io/en/stable/reference/dune-project/index.html