AGENTS.md: Align AI attribution policy with Kubernetes community

Adopt the policy established in kubernetes/community#8918: disclose AI
assistance in the PR description rather than via commit trailers.
Listing AI tooling as a co-author or using 'Assisted-by' / 'Co-developed-by'
trailers is now explicitly prohibited, matching the Kubernetes steering
committee's position on AI disclosure.

Further rationale: the Assisted-by schema (tool + model name) doesn't
capture common multi-agent patterns (planner + executor), and model
names alone don't tell reviewers much — the prompt harness and skills
matter just as much.

Also adds two new guidance sections:
- Pull request size: encourage splitting prep commits out separately
- Commit messages and text: remind agents that natural language can't
  be machine-checked, so humans should own the commit message prose

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

common/AGENTS.md

@@ -16,11 +16,43 @@ and the DCO check fails, tell the human to review
 the code and give them instructions on how to add
 a signoff.
 
-### Attribution
+### Attribution and AI disclosure
 
-When generating substantial amounts of code, you SHOULD
-include an `Assisted-by: TOOLNAME (MODELNAME)`. For example,
-`Assisted-by: Goose (Sonnet 4.5)`.
+Do NOT add an `Assisted-by`, `Co-developed-by`, or similar commit
+trailer crediting an AI tool. Instead, disclose AI assistance in the
+**pull request description**. Examples:
+
+- "I used a LLM to generate just unit tests."
+- "This code was written in part with the assistance of generative AI."
+- "A LLM was used to generate almost all of the code, but I am knowledgeable in this problem domain and reviewed it carefully."
+- "This code is generated, I am only partially knowledgable in this domain."
+- "Code is LLM generated; I don't know the progamming language but it did fix the problem."
+
+(The closer the commits are to being *entirely* AI, the more likely
+ it is that you should submit the PR as a draft, or even file an
+ issue first with a proposed design)
+
+If you're an agent generating a git commit, ensure your human sees
+this choice and preferably writes the text on their own.
+
+### Pull request size
+
+It is *very strongly* encouraged to split up "preparatory" commits
+that are independently reviewable from the main PR, and submit those separately.
+
+### Commit messages and text
+
+Software can be machine checked (via compilation and unit/integration tests)
+but natural languages like English cannot. It is strongly encouraged
+for you to write the commit message on your own, and to double check its
+accuracy.
+
+It is common in general for LLM-assisted/generated code to miss updating
+documentation or describe things incorrectly (especially after iteration).
+You should review these.
+
+When responding to review comments (and commenting in general) similarly
+that should be using your own written language by default.
 
 ## Code guidelines