Add agentpr pr edit subcommand

Update a PR's title and/or body as the agent user, sending only the
fields supplied.

internal/agent/gitea.go

@@ -214,6 +214,22 @@ func (c *GiteaClient) CreatePR(repoPath string, opts CreatePROptions) (PullReque
 	return pr, err
 }
 
+// EditPROptions are the fields an edit may change. Pointers so an unset field
+// is omitted from the payload entirely: Gitea overwrites whatever it is sent,
+// so a nil Title leaves the title alone while a pointer to "" clears it.
+type EditPROptions struct {
+	Title *string `json:"title,omitempty"`
+	Body  *string `json:"body,omitempty"`
+}
+
+// EditPR updates a pull request's title and/or body
+// (PATCH /api/v1/repos/{owner}/{repo}/pulls/{index}).
+func (c *GiteaClient) EditPR(repoPath string, number int, opts EditPROptions) (PullRequest, error) {
+	var pr PullRequest
+	err := c.do(http.MethodPatch, fmt.Sprintf("/api/v1/repos/%s/pulls/%d", repoPath, number), opts, &pr)
+	return pr, err
+}
+
 // GetPR fetches a single pull request.
 func (c *GiteaClient) GetPR(repoPath string, number int) (PullRequest, error) {
 	var pr PullRequest