test(agents): what a consumer's AGENTS.md gets, and what stays put

The merge on its own: the section is appended after one blank line,
merging again changes nothing, a moved section is rewritten where it
stands, and removing it gives the file its own bytes back. Markers
inside a fenced block, one quoting a fenced block of its own, are not
the section's. Unbalanced markers, a document that ends inside a code
block and notes that would break the section are refused, naming the
line.

End to end, through LoadProject on a staged fixture: app names lib
and has tool as host tooling; lib builds against base and has libtool
as its own. Transitive gives tool, lib and base in that order, and
neither app's own notes nor libtool's. A second update writes nothing,
mtime included. Direct drops base, edited notes replace the old ones
in the one section, and None restores the original bytes. A project
without an AGENTS.md does not get one, and a symlinked AGENTS.md is
written through the link, which stays a link. Through Run in process,
lint and run leave the file alone, while `test --list` and a build
that fails at the link both refresh it.

The fixture's AGENTS.md is checked in as AGENTS.md.in: an AGENTS.md in
this tree would read as instructions for the agents working on it.

Moving the CLI hook above the run and lint branches, or letting Direct
walk past the first loaded project, fails four of the checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/DependencyNotes/fixture/lib/AGENTS.consumer.md

@@ -0,0 +1,3 @@
+## lib
+
+Call lib through its module, never through its headers.

tests/DependencyNotes/fixture/lib/project.cpp

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+#pragma crafter dependency "../libtool"
+using namespace Crafter;
+
+// What app depends on. It builds against base, which app only reaches
+// through it. libtool is lib's own host tooling: it says nothing about using
+// lib, so its notes must not reach app.
+extern "C" Configuration CrafterBuildProject(std::span<const std::string_view>) {
+    Configuration cfg;
+    cfg.path = "./";
+    cfg.name = "lib";
+    cfg.outputName = "lib";
+    cfg.type = ConfigurationType::LibraryStatic;
+    // Loaded from lib/, so this resolves against it.
+    cfg.dependencies = { LocalProject({ .projectFile = "../base/project.cpp" }) };
+    return cfg;
+}