feat(#130): implement GitHub API methods and vcs types

- Add vcs package with shared review types (ReviewEvent, Review, ReviewRequest, ReviewComment, UserInfo)
- Add GetPullRequest, GetPullRequestDiff, GetPullRequestFiles, GetCommitStatuses to github/client.go
- Add GetFileContent, GetFileContentRef, ListContents to github/client.go
- Add doRequestWithBody helper for POST/PUT/DELETE operations
- Add review.go with PostReview, ListReviews, DeleteReview, DismissReview
- Add identity.go with GetAuthenticatedUser
- Remove TODO comment from github/client.go
- Add escapePath helper for URL path construction
- Add comprehensive tests for all new methods using httptest.NewServer

github/identity.go

@@ -0,0 +1,29 @@
+package github
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+)
+
+// userResponse is the GitHub API response for the authenticated user.
+type userResponse struct {
+	Login string `json:"login"`
+}
+
+// GetAuthenticatedUser returns the login of the currently authenticated user.
+func (c *Client) GetAuthenticatedUser(ctx context.Context) (string, error) {
+	reqURL := fmt.Sprintf("%s/user", c.baseURL)
+
+	body, err := c.doGet(ctx, reqURL)
+	if err != nil {
+		return "", fmt.Errorf("get authenticated user: %w", err)
+	}
+
+	var resp userResponse
+	if err := json.Unmarshal(body, &resp); err != nil {
+		return "", fmt.Errorf("parse user response: %w", err)
+	}
+
+	return resp.Login, nil
+}