feat: delete previous review before posting new one (#6)

Before posting a review, the bot now:
1. Calls GET /api/v1/user to identify its own login
2. Lists all reviews on the PR
3. Deletes any existing reviews from itself
4. Posts the fresh review

This keeps PR threads clean — one review per bot at any time.

New Gitea client methods:
- GetAuthenticatedUser() — token self-identification
- ListReviews() — fetch reviews on a PR
- DeleteReview() — delete a review by ID

Flag: --update-existing / UPDATE_EXISTING (default true)
Set to false to preserve old behavior (stack reviews).

All delete failures are non-fatal (logged as warnings).

Closes #6

gitea/client.go

@@ -266,3 +266,78 @@ func (c *Client) GetAllFilesInPath(ctx context.Context, owner, repo, path string
 	}
 	return results, nil
 }
+
+// Review represents a pull request review from the Gitea API.
+type Review struct {
+	ID   int64  `json:"id"`
+	User struct {
+		Login string `json:"login"`
+	} `json:"user"`
+	State string `json:"state"`
+	Stale bool   `json:"stale"`
+}
+
+// GetAuthenticatedUser returns the login name of the token's owner.
+func (c *Client) GetAuthenticatedUser(ctx context.Context) (string, error) {
+	reqURL := fmt.Sprintf("%s/api/v1/user", c.baseURL)
+	body, err := c.doGet(ctx, reqURL)
+	if err != nil {
+		return "", fmt.Errorf("get authenticated user: %w", err)
+	}
+	var user struct {
+		Login string `json:"login"`
+	}
+	if err := json.Unmarshal(body, &user); err != nil {
+		return "", fmt.Errorf("parse user response: %w", err)
+	}
+	if user.Login == "" {
+		return "", fmt.Errorf("empty login in user response")
+	}
+	return user.Login, nil
+}
+
+// ListReviews returns all reviews on a pull request.
+func (c *Client) ListReviews(ctx context.Context, owner, repo string, number int) ([]Review, error) {
+	reqURL := fmt.Sprintf("%s/api/v1/repos/%s/%s/pulls/%d/reviews",
+		c.baseURL,
+		url.PathEscape(owner),
+		url.PathEscape(repo),
+		number)
+	body, err := c.doGet(ctx, reqURL)
+	if err != nil {
+		return nil, fmt.Errorf("list reviews: %w", err)
+	}
+	var reviews []Review
+	if err := json.Unmarshal(body, &reviews); err != nil {
+		return nil, fmt.Errorf("parse reviews: %w", err)
+	}
+	return reviews, nil
+}
+
+// DeleteReview deletes a review by ID. The token must belong to the review author.
+func (c *Client) DeleteReview(ctx context.Context, owner, repo string, number int, reviewID int64) error {
+	reqURL := fmt.Sprintf("%s/api/v1/repos/%s/%s/pulls/%d/reviews/%d",
+		c.baseURL,
+		url.PathEscape(owner),
+		url.PathEscape(repo),
+		number,
+		reviewID)
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodDelete, reqURL, nil)
+	if err != nil {
+		return fmt.Errorf("create delete request: %w", err)
+	}
+	req.Header.Set("Authorization", "token "+c.token)
+
+	resp, err := c.http.Do(req)
+	if err != nil {
+		return fmt.Errorf("delete review: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		respBody, _ := io.ReadAll(resp.Body)
+		return fmt.Errorf("delete review failed (status %d): %s", resp.StatusCode, string(respBody))
+	}
+	return nil
+}