docs: add AGENTS.md as a symlink to README

Most coding agents have standardized on AGENTS.md as the file they load
automatically from the root of a repository. README already tells AI
tools to read and follow Documentation/process/coding-assistants.rst,
but that only helps if the agent decides to read README before acting,
which in practice it often doesn't.

Add a top-level AGENTS.md symlink to README, so agents load it on their
own and follow its pointer to the AI guidelines. This keeps a single
entry point for every reader, and avoids adding a separate copy of the
policy that could drift from the RST.

As an example, two different agents were asked to "create a new commit
renaming the release (in the Makefile) to AI Test".

Without AGENTS.md, the first agent added a Signed-off-by for the user on
its own. It should never have done that: only the human submitter can
certify the Developer Certificate of Origin, after reviewing the change.
It also used its own attribution tag instead of Assisted-by:

> Makefile: rename release to AI Test
>
> Co-Authored-By: <model name> <noreply@...>
> Signed-off-by: Sasha Levin <sashal@kernel.org>

The second agent added no attribution at all:

> Rename release to AI Test

With AGENTS.md in place, neither agent added a Signed-off-by, and both
used the Assisted-by tag. README itself mentions neither tag, so both
agents followed its pointer to coding-assistants.rst:

> Makefile: rename release to "AI Test"
>
> Change the release name from "Baby Opossum Posse" to "AI Test".
>
> Assisted-by: LLM

> Makefile: rename release to AI Test
>
> Assisted-by: LLM

Both agents also followed the kernel's general guidelines more closely.
The second agent's subject gained the "subsystem: summary phrase" form
that submitting-patches.rst asks for, instead of a bare "Rename release
to AI Test", and the first agent added a body describing the change.

Assisted-by: LLM
Signed-off-by: Sasha Levin <sashal@kernel.org>
Acked-by: Kees Cook <kees@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260924134945.3095661-1-sashal@kernel.org>

AGENTS.md

@@ -0,0 +1 @@
+README