feat(agents): keep dependencies' AGENTS.consumer.md in AGENTS.md

An agent working on a crafter-build project reads its AGENTS.md, and
learns nothing there about the libraries the project builds against
unless someone copied that in by hand. This repository shows the cost:
the code style section of its AGENTS.md restates what Crafter.LintRules
enforces, and goes stale whenever the rules change.

A library now ships those notes itself, as AGENTS.consumer.md beside
its project.cpp; nothing in its project.cpp changes. When a consumer
has an AGENTS.md beside its own project.cpp, `crafter-build` and
`crafter-build test` keep a section in it, between two marker comments,
holding its dependencies' notes. The section is appended the first time
and rewritten in place after that. The file is written only when the
section changes, and an AGENTS.md is never created. lint, format, run
and clean leave it alone. The update runs after loading, which has just
fetched every dependency, and before compiling, so a build that fails
still has the notes.

cfg.dependencyNotes picks the dependencies. Transitive, the default,
follows `dependencies` all the way down from the project's own
configurations, its tests and its `#pragma crafter dependency` tooling;
Direct stops at the first loaded project on each path; None removes
the section. A dependency's own pragma tooling never counts: it is
about how that dependency is built, not how it is used, and being no
`dependencies` edge it is never reached. Nor are a dependency's tests:
nothing builds them for the consumer, and their configurations can
point at a local their project file has since moved out of.

The walk has to know which Configurations belong to another project,
and where that project lives, and cfg.path cannot say: only the
Configuration GitProject and LocalProject return is rebased, so a
dependency's internal configurations keep "./", which resolves to the
consumer. The loader records it instead, LoadedProjectOf beside the
project cache, and keeps what the root's pragmas resolved to past the
frame LoadProject pops, PragmaDependencies. Neither is exported.

The markers count only as whole lines outside fenced code blocks, so
documentation can quote them. Unbalanced markers and a document ending
inside a code block leave the file alone with a warning: guessing which
part of someone's file is generated would overwrite their text, and
missing the section would add another one on every build. A note that
would break the section when it is read back, through a marker line of
its own or a code block it never closes, is left out with a warning.
The file is replaced through a temporary renamed onto the symlink's
target, so a linked AGENTS.md stays a link and a write cut short leaves
the old file rather than part of the new one.

The code is a new partition, Crafter.Build:Agents, listed in build.sh,
CrafterBuildModules, project.cpp and the umbrella.

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

AGENTS.md

@@ -29,6 +29,7 @@ Partitions and what they own:
 - `:Test` holds `AddTest`, `RunTests` and `TestRunner`.
 - `:Lint` is the lint/format driver plus the libclang token and AST layer.
 - `:Script` is the driver behind `crafter-build run`: it looks a name up in `cfg.scripts`, calls the callback and turns what it returns into the exit code.
+- `:Agents` keeps the dependency notes section of a project's AGENTS.md: it walks the dependencies `cfg.dependencyNotes` selects, reads each one's `AGENTS.consumer.md` and merges them between the section's markers. Which Configurations belong to another project, and where it lives, comes from the loader (`LoadedProjectOf` in `:External`, `PragmaDependencies` in `:Platform`), because only the Configuration `GitProject` and `LocalProject` return has its `path` rebased.
 - `:Progress` is console output. `:Shader` compiles shaders through glslang. `:Asset` is the hook into Crafter.Asset.
 
 ## Dev environment
@@ -188,3 +189,4 @@ Local builds are two scripts in `project.cpp`. Run them with the native binary:
 - A warning is printed only when its unit actually recompiles. A build with nothing to do is silent, which is not evidence that a warning is gone.
 - A std module source only compiles against the headers of its own libc++ release, and a sysroot does not always ship one (Alpine). The Flatpak therefore carries the sources of the release next to the bundled one under `share/crafter-build/libc++-modules/<major>/`, one `libcxx-<major>-modules` manifest module each. When the bundled LLVM is bumped, keep the release it leaves behind as such a module: sysroots on the stable distro stay on it for months. The native loop has no such directory; stage one by hand under `share/crafter-build/` to test against a foreign sysroot.
 - `VariantId()` names the output directory, so CI globs `bin/crafter.build-exe-<target>-<march>-*/` rather than predicting the hash. The hash includes the compiler, so after a toolchain update the previous variant's directories stay in `bin/` and `build/` until `clean`, and such a glob matches both.
+- The self-host build and `test` keep the dependency notes section of this very file. None of Crafter.Math, Crafter.Asset and Crafter.LintRules ships an `AGENTS.consumer.md` yet; once one does, building here appends the section. Commit it with the change that pulled it in.