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, 76 insertions, 0 deletions

diff --git a/test/test_fossil.ml b/test/test_fossil.ml
new file mode 100644
index 0000000..c4bb56f
--- /dev/null
+++ b/test/test_fossil.ml
@@ -0,0 +1,76 @@
+(*─────────────────────────────────────────────────────────────────────────────┐
+│ SPDX-FileCopyrightText: 2026 toastal <https://toast.al/contact/>             │
+│ SPDX-License-Identifier: LGPL-2.1-or-later WITH OCaml-LGPL-linking-exception │
+└─────────────────────────────────────────────────────────────────────────────*)
+open Alcotest
+open Nixtamal
+
+let suite =
+	let t = Input.Template.make in
+	[
+		test_case "Manifest Fossil reference check-in from KDL" `Quick (fun () ->
+			let reference =
+				let kdl =
+					{|check-in "abc123"|}
+					|> KDL.of_string
+					|> Result.get_ok
+				in
+				match Manifest.Fossil.Reference.codec.of_kdl kdl with
+				| Ok ref -> ref
+				| Error err -> failwith Fmt.(str "%a" (list ~sep: semi KDL.Valid.pp_err) err)
+			in
+			check
+				(testable Input.Fossil.Reference.pp Input.Fossil.Reference.equal)
+				"check-in is decoded as Fossil reference"
+				(`Checkin "abc123")
+				reference
+		);
+		test_case "Manifest Fossil input roundtrip" `Quick (fun () ->
+			let fossil =
+				Manifest.Fossil.make
+					~repository:(t "https://example.org/src.fossil")
+					~reference:(`Branch "trunk")
+			in
+			let roundtrip =
+				fossil
+				|> Manifest.Fossil.codec.to_kdl
+				|> Manifest.Fossil.codec.of_kdl
+				|> Result.get_ok
+			in
+			check
+				(testable Manifest.Fossil.pp Manifest.Fossil.equal)
+				"Fossil KDL codec is shape-preserving"
+				fossil
+				roundtrip
+		);
+		test_case "Lockfile Fossil kind roundtrip" `Quick (fun () ->
+			let input_kind =
+				Input.make_kind_fossil
+					~repository:(t "https://example.org/src.fossil")
+					~reference:(`Tag "v1.0")
+					~date:"2026-01-01T00:00:00Z"
+					~latest_checkin:"abc123"
+					()
+			in
+			let models = Input.jg_models2 @@ Input.make ~name:(Name.Name.make "fossil") ~kind:input_kind () in
+			let lock_kind = Lockfile.Kind.to_lock ~models input_kind in
+			let decoded =
+				lock_kind
+				|> Jsont.Json.encode Lockfile.Kind.jsont
+				|> Result.get_ok
+				|> Jsont.Json.decode Lockfile.Kind.jsont
+				|> Result.get_ok
+			in
+			check
+				(testable Lockfile.Kind.pp Lockfile.Kind.equal)
+				"Fossil lockfile kind JSON codec is shape-preserving"
+				lock_kind
+				decoded;
+			match lock_kind with
+			| `Fossil fossil_lock ->
+				check int "Fossil mirrors are always empty" 0 (List.length fossil_lock.mirrors);
+				check (option string) "Fossil lock datetime" (Some "2026-01-01T00:00:00Z") fossil_lock.datetime;
+				check (option string) "Fossil lock latest_checkin" (Some "abc123") fossil_lock.latest_checkin
+			| _ -> fail "Expected Fossil lock kind"
+		);
+	]