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, 2 insertions, 1 deletions

diff --git a/nixtamal.opam b/nixtamal.opam
index 0904ad8..17b3e83 100644
--- a/nixtamal.opam
+++ b/nixtamal.opam
@@ -9,7 +9,7 @@ tags: ["nix"]
 depends: [
   "dune" {>= "3.20"}
   "camomile"
-  "cmdliner" {>= "2.0.0"}
+  "cmdliner"
   "eio"
   "eio_main"
   "fmt"
@@ -23,6 +23,7 @@ depends: [
   "alcotest" {with-test}
   "qcheck" {with-test & >= "0.91"}
   "qcheck-alcotest" {with-test & >= "0.91"}
+  "bisect_ppx" {with-test}
   "odoc" {with-doc}
 ]
 build: [