fix: timer leak and http field shadowing in github client

- Add timer.Stop() to the timer.C branch to prevent timer leak on the
  normal path (previously only called in ctx.Done branch)
- Rename struct field 'http' to 'httpClient' to avoid shadowing the
  net/http import

Addresses self-review findings on PR #110.

github/client.go

@@ -95,7 +95,7 @@ type Client struct {
 	// accepting full URLs instead.
 	baseURL string
 	token   string
-	http    *http.Client
+	httpClient *http.Client
 
 	// retryBackoff defines the delays between retry attempts for 429 responses.
 	// retryBackoff[i] is the delay before attempt i+1 (after attempt i fails).
@@ -117,7 +117,7 @@ func NewClient(token, baseURL string) *Client {
 	return &Client{
 		baseURL: strings.TrimRight(baseURL, "/"),
 		token:   token,
-		http:    &http.Client{Timeout: 30 * time.Second},
+		httpClient: &http.Client{Timeout: 30 * time.Second},
 		now:     time.Now,
 	}
 }
@@ -125,7 +125,7 @@ func NewClient(token, baseURL string) *Client {
 // SetHTTPClient sets the underlying HTTP client used for requests.
 // This is intended for testing to inject mock transports.
 func (c *Client) SetHTTPClient(hc *http.Client) {
-	c.http = hc
+	c.httpClient = hc
 }
 
 // SetRetryBackoff sets the delays between retry attempts.
@@ -192,6 +192,7 @@ func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept st
 				timer := time.NewTimer(delay)
 				select {
 				case <-timer.C:
+					timer.Stop()
 				case <-ctx.Done():
 					timer.Stop()
 					return nil, ctx.Err()
@@ -210,7 +211,7 @@ func (c *Client) doRequest(ctx context.Context, method, reqURL string, accept st
 			req.Header.Set("Accept", "application/vnd.github+json")
 		}
 
-		resp, err := c.http.Do(req)
+		resp, err := c.httpClient.Do(req)
 		if err != nil {
 			return nil, fmt.Errorf("do request: %w", err)
 		}