[misc] add AGENTS.md file (#7014)

## Describe your changes

[#7009](https://github.com/netbirdio/netbird/pull/7009) wrote down what
we expect
from a contribution: an agreed ticket first, a change the author has
run, one
purpose per PR, small enough to review, a title tag CI already enforces.
That
  works for contributors who read CONTRIBUTING.md.

A growing share of what reaches the queue is drafted by a coding agent,
and those
tools never read it. The result is predictable and repetitive: a PR with
no ticket
behind it, an approach we would have talked the author out of in five
minutes, a
diff too large to review carefully against a daemon that runs as root, a
description longer than the code it describes, and an author who cannot
answer
questions about their own change. That is not a tooling problem. It is
what
happens when a capable tool is pointed at a repository whose
expectations nobody
  told it.

AGENTS.md states those expectations in the format agents pick up
automatically.
The first entry in its stop-and-ask list is asking the contributor for
the
discussion or issue before drafting anything, which is where most of
these PRs
go wrong. It also carries the repository map, the Go conventions we
apply in
review, the local verification commands, the PR template and title-tag
rules,
and instructions to read the review bots rather than resolve their
threads.

CONTRIBUTING.md gains a short section saying plainly that we have no
policy for
or against agents, that this guide exists because of what we keep seeing
rather
than as a restriction on tools, and that whatever produced a diff its
author owns
it. It also records that we weigh whether a contribution is worth
maintaining,
and that what gets merged has to match our security and design
expectations.

CLAUDE.md is a one-line pointer rather than a symlink, deliberately. A
symlink is
tidier, but a Windows checkout without symlink support materialises it
as a plain
file containing the target path, and an agent then reads one word as its
entire
instruction set with no error to show for it. Given how much Windows
work this
repository sees, a pointer file that survives every checkout is worth
the extra
  file.

Nothing here is enforced by CI, and no workflow changes. It is guidance,
aimed at
  PRs arriving in a reviewable state more often.

CONTRIBUTING.md

@@ -66,11 +66,41 @@ Typical bug fixes, internal refactors, documentation updates, and tests do not
 need a design discussion, but should still be tied to an issue so the work is
 visible and nobody duplicates it.
 
+### Using AI coding agents
+
+We have no policy for or against using an AI agent to write NetBird code. That
+choice is yours, and we are not going to interrogate anyone about their tools.
+
+What we do have is a lot of incoming contributions that were plainly drafted with
+one, and enough experience reviewing them to see the same avoidable problems
+again and again: no ticket behind the change, a diff far too large to review, a
+description longer than the code it describes, an approach that was never going
+to be accepted, and an author who cannot answer questions about their own PR.
+None of that is caused by the tooling — it is what happens when a tool is pointed
+at a repository whose expectations it has never been told.
+
+So rather than a rule, there is a guide. [AGENTS.md](AGENTS.md) restates the
+expectations from this document in the form agents read automatically
+(`CLAUDE.md` points to it), so pointing your tool at the repository is usually
+enough. Among other things it tells the agent to ask you for the
+discussion or issue before drafting a PR, to keep the change small and
+single-purpose, to run the tests locally, to use this repository's PR template
+and title tags, and to write a description a reviewer can get through.
+
+The guardrails are the point, and they are the same ones we apply to everyone: an
+agreed ticket, a change you have actually run, a diff small enough to review with
+care, and an author who can explain it. Whatever wrote the diff, you are its
+author — you own every line you submit and the consequences of opening a PR with it.
+
+We may assess whether a contribution is maintainable and whether its merged code
+aligns with our security standards and design expectations.
+
 ## Contents
 
 - [Contributing to NetBird](#contributing-to-netbird)
     - [Ticket first, PR second](#ticket-first-pr-second)
         - [High-risk areas](#high-risk-areas)
+        - [Using AI coding agents](#using-ai-coding-agents)
     - [Contents](#contents)
     - [Code of conduct](#code-of-conduct)
     - [Directory structure](#directory-structure)