docs(contributing): clarifying and expanding AI requirements

Introduce stricter requirements regarding AI-powered contributions, as
inspired by the Linux Kernel's guidelines.

CONTRIBUTING.md

@@ -73,7 +73,44 @@ adhere to the following requirements:
    (bug reports, feature requests, pull request descriptions,
    responding to humans, ...).
 
-For more info, please refer to the [AGENTS.md](AGENTS.md) file.
+### Attribution
+
+When using AI assistance in contributions:
+
+- **AI cannot be a commit author.** All commits must be authored by a
+  human contributor.
+- **AI cannot sign off commits.** Only humans can legally certify
+  commits by adding a `Signed-off-by:` tag. AI tools MUST NOT add
+  `Signed-off-by` tags.
+- **The human author bears full responsibility.** The human
+  contributor is responsible for:
+  - Reviewing all AI-generated or AI-assisted code
+  - Ensuring compliance with licensing requirements
+  - Taking full responsibility for the contribution
+- **AI-assisted commits must include an `Assisted-by:` footer**. The
+  format is:
+
+  ```
+  Assisted-by: AGENT_NAME:MODEL_VERSION [TOOL1] [TOOL2]
+  ```
+
+  Where:
+  - `AGENT_NAME` is the name of the AI tool or framework
+  - `MODEL_VERSION` is the specific model version used
+  - `[TOOL1] [TOOL2]` are optional specialized analysis tools used
+    (not basic tools like git, cargo, Nix, editors)
+
+  Example:
+
+  ```
+  Assisted-by: Claude:claude-3-sonnet
+  ```
+
+  See the [AGENTS.md](AGENTS.md#attribution) file for the full format
+  specification.
+
+For more info, please refer to the [AGENTS.md](AGENTS.md)
+file.
 
 ## Code of Conduct