feat(tools): pr_files_diff with caps

Returns per-file unified diff for a PR, capped at 20KB/file and 200KB
total response. Files exceeding per-file cap report truncated+omitted_lines;
files that would push the response over 200KB go to omitted_files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/gitea/pulls.go

@@ -64,3 +64,40 @@ func (c *Client) GetPullRequest(ctx context.Context, owner, repo string, index i
 	}
 	return &pr, nil
 }
+
+type PullRequestFile struct {
+	Filename  string `json:"filename"`
+	Status    string `json:"status"` // added | modified | deleted | renamed
+	Additions int    `json:"additions"`
+	Deletions int    `json:"deletions"`
+}
+
+func (c *Client) GetPullRequestFiles(ctx context.Context, owner, repo string, index int) ([]PullRequestFile, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/pulls/%d/files", owner, repo, index)
+	body, status, err := c.GetJSON(ctx, p)
+	if err != nil {
+		return nil, err
+	}
+	if err := MapStatus(status, body); err != nil {
+		return nil, err
+	}
+	var files []PullRequestFile
+	if err := json.Unmarshal(body, &files); err != nil {
+		return nil, err
+	}
+	return files, nil
+}
+
+// GetPullRequestDiff returns the raw unified diff. The endpoint serves text/plain, not JSON,
+// so we use doRaw to bypass the json Accept header expectation.
+func (c *Client) GetPullRequestDiff(ctx context.Context, owner, repo string, index int) ([]byte, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/pulls/%d.diff", owner, repo, index)
+	resp, err := c.doRaw(ctx, "GET", p, nil)
+	if err != nil {
+		return nil, err
+	}
+	if err := MapStatus(resp.Status, resp.Body); err != nil {
+		return nil, err
+	}
+	return resp.Body, nil
+}