Add a policy for LLM-generated content

AI_POLICY.md

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+TigerVNC AI policy
+==================
+
+The legal and ethical status of LLM-generated content is currently
+uncertain. LLMs are trained on large amounts of existing work, much of
+it under licenses that require attribution or impose other conditions,
+and their output can reproduce or be derived from that work without any
+indication of its origin. TigerVNC is a Free Software project that
+depends on copyright and licenses being respected, and we want to show
+the same respect for the work of others. For now, the TigerVNC project
+has therefore decided to take a cautionary approach to such content.
+This policy reflects the current situation, and will be revisited as
+it becomes clearer.
+
+Any significant contribution to TigerVNC must not include LLM-generated
+content or be derived from LLM-generated content. This applies to all
+contributions, including code, documentation and translations.
+
+Minor LLM-generated contributions can still be accepted, as long as they
+fulfill all other requirements on quality. The commits must be marked
+with an "Assisted-by:" tag for transparency.
+
+All contributions must be submitted by a human who has reviewed them,
+understands them, and is able to answer questions about them.
+
+There is no strict definition of what is a "significant" contribution.
+The decision will be made by the TigerVNC maintainer or maintainers that
+review the contribution. A guideline is the GNU project's definition[^1]
+of legally significant changes.
+
+As an exception, significant LLM-generated contributions to test cases
+can be accepted. Tests are largely constrained by the code they
+exercise, leaving less room for reproducing the work of others, and
+they can be removed without affecting TigerVNC itself. Such changes
+must be kept in separate commits, marked with an "Assisted-by:" tag, so
+that they can be identified and removed if needed in the future.
+
+Other uses of AI
+----------------
+
+This policy covers content included in contributions. It does not cover
+using AI tools for research, understanding the code, finding bugs,
+debugging, or as support when reviewing, as long as the output is not
+included in a contribution. Neither does it cover assistive uses such
+as spelling and grammar checking, or screen readers. Bug reports and
+comments written with AI assistance should be verified by the contributor
+before posting.
+
+[^1]: https://www.gnu.org/prep/maintain/maintain.html#Legally-Significant