Add an agents.md providing instructions on writing code.

Note: I've generally been assuming that the agent can follow the style
of existing code. So every line in this is something that I've
specifically seen agents get wrong multiple times.

Change-Id: Id6b6d4ff002a342de80233a95e5118f46a6a6964
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/23480
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: Matt Stark <msta@google.com>

.agents/AGENTS.md

diff --git a/.agents/AGENTS.md b/.agents/AGENTS.md
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+# Rust Style Guidelines & Best Practices
+
+Please adhere to the following conventions and best practices when writing or
+modifying Rust code in this repository:
+
+## Code Structure & Formatting
+
+* **Code Ordering**: Be consistent about the layout within a file.
+  * The structure should generally be:
+    1. `mod` statements
+    2. `use` statements
+    3. Core code (structs, enums, impls, functions)
+    4. Test module at the bottom: `#[cfg(test)]`
+  * Structs should always come immediately before their impl and impl traits.
+  * Full types should be defined before reference types (e.g. if we were
+    writing the standard library, String before str)
+* **Inlining**: If you only use a variable once, prefer inlining it rather
+  than assigning it to a separate binding.
+* **Collect**: Prefer using `.collect()` over manually iterating and
+  adding/pushing to data structures when possible.
+* **Unused Variables**: Variables starting with `_` (e.g. `_var`) should only
+  be used if they are truly never used. If you begin using a variable that was
+  prefixed with an underscore, rename it to remove the underscore prefix.
+* **Unsafe Code**: All `unsafe` blocks must be prefixed with a
+  `// Safety: <reason>` comment explaining why the usage is safe.
+* **Warnings**: The code (including the tests) should compile with no warnings
+  and no clippy lint errors.
+
+## Imports & Use Statements
+
+* **Test-only Imports**: Imports that are only used inside tests should be
+  moved into a module block guarded by `#[cfg(test)]`.
+
+## Documentation
+
+* Anything marked as pub should have a docstring (starts with `///`).
+
+## Coding guidelines
+
+* Data members should not be public without good reason
+* **FFI Boundary**: No FFI function calls (such as `extern "C"` blocks or raw
+  FFI calls) are allowed outside of the `ffi` submodule. The only exception is
+  `intern_string`.
+* **No Statics**: Do not use global static variables or `thread_local!`
+  structures. They hinder concurrency, unit testing isolation, and
+  multi-session safety. Always prefer passing state and context explicitly via
+  arguments or trait objects.
+* **Dynamic dispatch**: Never use dynamic dispatch (`dyn Trait`) in the hot
+  path (any code that could be called an arbitrary number of times by a
+  starlark rule).
+  * Avoid using dynamic dispatch (`dyn Trait`) in general if you can, but
+    you may use it to increase testability of code if it is not in the hot
+    path.
+    * Eg. Using `dyn Trait` to support a "real" version in production code and
+      a fake one in test code.
+* `collect_repr` and `collect_str`
+  * collect_repr should be defined on starlark types iff `Debug` is
+    available on a type. It should just `write!(collector, "{:?}").unwrap()`
+  * collect_str should be defined on starlark types iff `Display` is
+    available on a type. It should just `write!(collector, "{}").unwrap()`
+* **Error Handling**: Prefer using custom error enums with `thiserror` for
+  crate-level errors, rather than generic strings or custom hand-written
+  Display implementations. Let exceptions and failures bubble up naturally.
+* **Crate Encapsulation**: Keep modules, structs, and fields private or
+  `pub(crate)` by default to maintain clean boundaries. Only use `pub` for APIs
+  that are intended to be consumed by other crates.
+
+# C++ Style guidelines and best practices
+
+* Never use `#pragma once` - use header guards instead.