docs: require AI attribution footer on comments, not commits

[andig]

Document the AI attribution policy so it is unambiguous: AI-authored GitHub comments, reviews and PR/issue descriptions carry the Claude Code footer, while commit messages stay clean. This replaces the previous blanket "no generator footers" note, which conflicted with it.

• new AGENTS.md § "AI Attribution"
• footer required on comments, reviews and PR/issue bodies
• commit messages omit it (and still carry no co-author trailers)

🤖 Generated with Claude Code

[sourcery-ai]

Hey - I've left some high level feedback:

• Consider clarifying what counts as "authored by an AI agent" (e.g., fully generated vs. partially edited by a human) so contributors know when the footer is required.
• It may be helpful to state whether the footer should be added when editing an existing human-authored comment/PR description that an AI later modifies, or only for newly created AI-authored content.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider clarifying what counts as "authored by an AI agent" (e.g., fully generated vs. partially edited by a human) so contributors know when the footer is required.
- It may be helpful to state whether the footer should be added when editing an existing human-authored comment/PR description that an AI later modifies, or only for newly created AI-authored content.

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[naltatis]

@andig why would we want to add this in PR description? As of right now I expect the majority of PRs to be AI-generated or assisted.

[andig]

I would prefer to be honest: if I have Claude post in my name then I'm responsible. It still posts things that I do not 100% understand but felt obliged to move forward (see #30878 (comment) for example). This is still AI-generated content and I'd prefer to make that transparent.