docs/process: document AI-assisted patch tags

Add documentation for the Assisted-by tag to clarify how
AI-assisted tooling may be disclosed in Xen patches.

The guidance follows the Linux kernel documentation.

Signed-off-by: Cody Zuschlag <cody.zuschlag@gmail.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>

docs/process/sending-patches.pandoc

@@ -195,6 +195,28 @@ E.g.:
 
     Requested-by: Jane Doe <jane.doe@example.org>
 
+### Assisted-by:
+
+When AI tools contribute to Xen development, proper attribution
+helps track the evolving role of AI in the development process.
+Contributions should include an `Assisted-by:` tag in the
+following format:
+
+    Assisted-by: AGENT_NAME:MODEL_VERSION [, ...]
+
+Where:
+
+* `AGENT_NAME` is the name of the AI tool or framework
+* `MODEL_VERSION` is the specific model version used
+
+Basic development tools (git, gcc, make, editors) should not be listed.
+Specialised but deterministic tools may optionally be listed, but their use
+should be clear from other context in the commit message.
+
+Example:
+
+    Assisted-by: Claude:claude-3-opus
+
 ### Signed-off-by:
 
 This mandatory tag specifies the author(s) of a patch (for each author a
@@ -209,6 +231,15 @@ E.g.:
 The author must be a natural person (not a team or just a company) and the
 `Signed-off-by:` tag must include the real name of the author (no pseudonym).
 
+AI agents MUST NOT add `Signed-off-by:` tags. Only humans can legally
+certify the Developer Certificate of Origin (DCO). The human submitter
+is responsible for:
+
+* Reviewing all AI-generated code
+* Ensuring compliance with licensing requirements
+* Adding their own `Signed-off-by:` tag to certify the DCO
+* Taking full responsibility for the contribution
+
 By signing the patch with her/his name the author explicitly confirms to have
 made the contribution conforming to the `Developer's Certificate of Origin`: