feat: add GetChangedFiles() method to Gitea client for PR file diffs

Add ChangedFile type and GetChangedFiles() method that calls the Gitea
API endpoint GET /repos/{owner}/{repo}/pulls/{index}/files to retrieve
the list of files changed in a pull request. This is a prerequisite for
displaying changed files in the PR detail view (#189).

Includes unit tests for success and error cases.

Closes leeworks-agents/gitea-mobile#205

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

internal/gitea/client.go

@@ -760,6 +760,33 @@ func (c *Client) GetPull(ctx context.Context, token, owner, repo string, index i
 	return &pr, nil
 }
 
+// ChangedFile represents a file changed in a pull request.
+type ChangedFile struct {
+	Filename    string `json:"filename"`
+	Status      string `json:"status"` // "added", "modified", "removed", "renamed"
+	Additions   int    `json:"additions"`
+	Deletions   int    `json:"deletions"`
+	Changes     int    `json:"changes"`
+	PreviousFilename string `json:"previous_filename,omitempty"`
+}
+
+// GetChangedFiles fetches the list of files changed in a pull request.
+func (c *Client) GetChangedFiles(ctx context.Context, token, owner, repo string, index int64) ([]ChangedFile, error) {
+	path := fmt.Sprintf("/repos/%s/%s/pulls/%d/files?limit=50", owner, repo, index)
+	resp, err := c.doRequest(ctx, token, http.MethodGet, path, nil)
+	if err != nil {
+		return nil, fmt.Errorf("fetching changed files: %w", err)
+	}
+	defer resp.Body.Close()
+
+	var files []ChangedFile
+	if err := json.NewDecoder(resp.Body).Decode(&files); err != nil {
+		return nil, fmt.Errorf("decoding changed files: %w", err)
+	}
+
+	return files, nil
+}
+
 // GetIssueComments fetches comments for an issue or pull request.
 func (c *Client) GetIssueComments(ctx context.Context, token, owner, repo string, index int64) ([]Comment, error) {
 	path := fmt.Sprintf("/repos/%s/%s/issues/%d/comments?limit=50", owner, repo, index)