Merge branch 'claude/dependency-notes': a library's AGENTS.consumer.md in its consumers' AGENTS.md

Brings in dependency notes: a crafter-build project that ships an
AGENTS.consumer.md beside its project.cpp has it kept in a marked
section of each consumer's AGENTS.md, refreshed by `crafter-build` and
`crafter-build test`, chosen by cfg.dependencyNotes (Transitive,
Direct, None). The new partition is Crafter.Build:Agents; the loader
records where each GitProject and LocalProject came from and what a
project file's pragmas resolved to.

The branch started before master's four external-fetch commits. Both
touch External.cpp, the Clang partition, project.cpp, README.md and
AGENTS.md, but in different places, so the merge needed no hand
reconciliation; the merged tree passes build.sh, the self-host build,
format --check and the test suite.

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, a build here can change AGENTS.md whenever that repository changes its notes, since all three are unpinned. The tree is then dirty, so the next build's `git describe --dirty` version differs, which is a new `crafter.build-lib` variant and a full rebuild. Commit the section to settle it.