feat(agents): write AGENTS.md from a tracked AGENTS.template.md

The dependency notes were kept in a section of the AGENTS.md beside
project.cpp, the file a project commits, so a build could change a
tracked file at any time: a dependency editing its AGENTS.consumer.md,
an unpinned one moving on, a flag pulling a dependency in or out, each
showed up in the consumer's diff as a change nobody made. Users
reported it as the feature clobbering their diffs.

A project now commits its instructions as AGENTS.template.md, and a
build and a test run write AGENTS.md from it: a first line saying the
file is generated and where to edit, the template's text as written,
then the notes section, after it or where the template's marker lines
put it. AGENTS.md is build output and goes into .gitignore. It is
created when missing, since a fresh clone has none, and otherwise
written as before: only when its text changes, in place and under a
lock, so links, ownership and a read-only file are respected.

The first line is also how a build tells its own AGENTS.md from one
someone wrote. Beside a template the second is easily there, as a
committed AGENTS.md nobody has removed yet; it is never replaced, and
each build warns until it is deleted. An empty file counts as a
build's, since that is what a write cut short leaves of a new one. An
AGENTS.md linked to the template is refused: writing it would write
the committed file again. A template the notes cannot be placed in
leaves no empty AGENTS.md behind.

The in-place mode is removed rather than kept as the fallback for a
project without a template. It is the default that put generated text
into tracked files unasked, the moment any dependency started shipping
notes. Without a template a build no longer touches AGENTS.md. One
that still holds a section from 3.0.0 would go stale silently, under a
preamble promising that every build rewrites it, so it gets a warning
naming the move instead.

MergeDependencyNotes keeps its rules and now runs on the template. The
test changes in this commit rather than its own, because it pinned the
in-place behaviour this removes: it now covers the generated file's
shape, edits and markers in the template, the ownership check, a CRLF
template, a link to the template and the cases without one.

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

AGENTS.md

@@ -29,7 +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.
+- `:Agents` writes a project's AGENTS.md from its AGENTS.template.md: it walks the dependencies `cfg.dependencyNotes` selects, reads each one's `AGENTS.consumer.md` and merges them into the template's dependency notes section. 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
@@ -194,4 +194,4 @@ Local builds are two scripts in `project.cpp`. Run them with the native binary:
 - The wasm compile flags live in one place, `WasmBaselineFlags` in `Crafter.Build-Clang.cppm`: the std module and every unit have to agree on them, and `VariantId()` and the std PCM directory hash them, so a change rebuilds instead of mixing old and new BMIs. They enable C++ exceptions, so the link takes the exception-enabled libc++ and libunwind from the sysroot's `lib/<triple>/eh/` (wasi-sdk 33 and later). The Flatpak's sysroot has them and a distro's may not; where they are missing the wasm tests skip, so run those through the Flatpak. A browser page checks the engine with the `WasmBaselineProbes` named in `variants.json`.
 - `WasiBrowserRuntimeExit` runs `wasi-runtime/runtime.js` under node and skips where there is none. The Flatpak has no node, so CI never runs it: after touching `runtime.js`, run the suite in the native loop with node on `PATH`.
 - `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.
+- This repository has no `AGENTS.template.md`, so the self-host build and `test` leave this file alone, and the `AGENTS.consumer.md` Crafter.Math ships does not reach it. A template would make this file build output: it would go into `.gitignore`, and a fresh clone or worktree would have no AGENTS.md until its first build.