feat(github): implement Reviewer and Identity interfaces (#81)

Implement the remaining vcs.Client interface methods for github.Client:

Reviewer:
- PostReview: POST /repos/{owner}/{repo}/pulls/{number}/reviews
- ListReviews: GET /repos/{owner}/{repo}/pulls/{number}/reviews
  with state translation (CHANGES_REQUESTED → REQUEST_CHANGES, etc.)
- DeleteReview: DELETE /repos/{owner}/{repo}/pulls/{number}/reviews/{id}
  Returns ErrCannotDeleteSubmittedReview on 422
- DismissReview: PUT /repos/{owner}/{repo}/pulls/{number}/reviews/{id}/dismissals

Identity:
- GetAuthenticatedUser: GET /user

Infrastructure:
- Add doRequestWithBody helper for POST/PUT/DELETE with JSON bodies
- Update conformance_test.go: var _ vcs.Client = (*github.Client)(nil)

All unit tests pass including error cases (401, 404, 422, malformed).

github/identity_test.go

@@ -0,0 +1,46 @@
+package github
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"testing"
+)
+
+func TestGetAuthenticatedUser_HappyPath(t *testing.T) {
+	c := newTestClient(t, func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != "GET" {
+			t.Errorf("expected GET, got %s", r.Method)
+		}
+		if r.URL.Path != "/user" {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+		if r.Header.Get("Authorization") != "Bearer test-token" {
+			t.Errorf("unexpected auth header: %s", r.Header.Get("Authorization"))
+		}
+		json.NewEncoder(w).Encode(map[string]string{"login": "review-bot"})
+	})
+
+	login, err := c.GetAuthenticatedUser(context.Background())
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if login != "review-bot" {
+		t.Errorf("expected login 'review-bot', got %q", login)
+	}
+}
+
+func TestGetAuthenticatedUser_401(t *testing.T) {
+	c := newTestClient(t, func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(401)
+		w.Write([]byte(`{"message":"Bad credentials"}`))
+	})
+
+	_, err := c.GetAuthenticatedUser(context.Background())
+	if err == nil {
+		t.Fatal("expected error for 401")
+	}
+	if !IsUnauthorized(err) {
+		t.Errorf("expected IsUnauthorized=true, got error: %v", err)
+	}
+}