docs: require underlying motivation in agent-created PRs

The existing `AGENTS.md` guidance requires PRs to describe the problem, but only commit bodies explicitly require motivation. Port the PR motivation requirement from the Gitoxide worktree so reviewers can understand why an agent pursued the work.

Require agent-created PR descriptions to identify the underlying motivation, such as the benefiting project or where the bug was discovered. Clarify that restating the fix is insufficient.

Validation: reviewed the wording against the Gitoxide requirement and ran `git diff --check`. No tests were run because this changes repository instructions only.

Author: codex

AGENTS.md

@@ -9,6 +9,12 @@ when communicating through a person's account.
 issue with a fix attached. Do not create an issue solely to follow it immediately
 with a PR that fixes it; describe the problem in the PR instead.
 
+Agents creating a PR must state the underlying motivation in its description, such
+as the project that will benefit or where the bug was first discovered. Any
+motivation is valid; explain why the work was pursued rather than restating what
+the PR does. "Want to fix a bug" is insufficient because that is already evident
+from the PR itself.
+
 # Gix backend implementations
 
 Gix backend operations must use Gix through the supported GixPython (`gix`)