feat: add MergePull() method to Gitea client

Add MergePull() that calls POST /repos/{owner}/{repo}/pulls/{index}/merge
with the specified merge style (merge, rebase, rebase-merge, squash).
Defaults to "merge" if no style specified. Includes unit tests for
success, default style, and error cases.

This is a prerequisite for #177 (merge PR button in UI) and #206
(POST /pulls merge handler).

Closes leeworks-agents/gitea-mobile#187

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/gitea/client.go

@@ -972,6 +972,40 @@ func (c *Client) SetIssueState(ctx context.Context, token, owner, repo string, i
 	return nil
 }
 
+// MergePull merges a pull request using the specified merge style.
+// Valid styles: "merge", "rebase", "rebase-merge", "squash".
+// If style is empty, defaults to "merge".
+func (c *Client) MergePull(ctx context.Context, token, owner, repo string, index int64, style, title, message string) error {
+	if style == "" {
+		style = "merge"
+	}
+
+	payload := map[string]string{
+		"Do": style,
+	}
+	if title != "" {
+		payload["merge_message_field"] = title
+	}
+	if message != "" {
+		payload["merge_message_field"] = message
+	}
+
+	jsonData, err := json.Marshal(payload)
+	if err != nil {
+		return fmt.Errorf("marshaling merge request: %w", err)
+	}
+
+	path := fmt.Sprintf("/repos/%s/%s/pulls/%d/merge", owner, repo, index)
+	resp, err := c.doRequest(ctx, token, http.MethodPost, path, strings.NewReader(string(jsonData)))
+	if err != nil {
+		return fmt.Errorf("merging pull request: %w", err)
+	}
+	resp.Body.Close()
+
+	c.InvalidateAll()
+	return nil
+}
+
 // AddComment creates a comment on an issue and returns the created Comment.
 func (c *Client) AddComment(ctx context.Context, token, owner, repo string, index int64, body string) (*Comment, error) {
 	return c.PostComment(ctx, token, owner, repo, index, body)