fix(vcs): thread CommitID through abstraction layer (#114)

Add CommitID field to vcs.ReviewRequest so the commit anchor propagates
through the vcs.Client interface to platform adapters.

Changes:
- vcs/types.go: Add CommitID string field to ReviewRequest
- gitea/client.go: Add commitID parameter to PostReview, include in API payload
- gitea/adapter.go: Pass req.CommitID to underlying client
- github/review.go: Use req.CommitID as primary, fall back to comment-level
- cmd/review-bot/main.go: Set CommitID on ReviewRequest from pr.Head.SHA

Fixes #114

gitea/client_test.go

@@ -135,7 +135,7 @@ func TestPostReview(t *testing.T) {
 	defer server.Close()
 
 	client := NewClient(server.URL, "test-token")
-	review, err := client.PostReview(context.Background(), "owner", "repo", 3, "APPROVED", "LGTM", nil)
+	review, err := client.PostReview(context.Background(), "owner", "repo", 3, "APPROVED", "LGTM", "", nil)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -182,7 +182,7 @@ func TestPostReview_Non200(t *testing.T) {
 	defer server.Close()
 
 	client := NewClient(server.URL, "test-token")
-	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "APPROVED", "test", nil)
+	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "APPROVED", "test", "", nil)
 	if err == nil {
 		t.Fatal("expected error for 403, got nil")
 	}
@@ -1144,3 +1144,62 @@ func TestSanitizeErrorForLog(t *testing.T) {
 		})
 	}
 }
+
+func TestPostReview_CommitID_InPayload(t *testing.T) {
+	var gotPayload struct {
+		Body     string `json:"body"`
+		Event    string `json:"event"`
+		CommitID string `json:"commit_id"`
+	}
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewDecoder(r.Body).Decode(&gotPayload)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(200)
+		json.NewEncoder(w).Encode(map[string]any{
+			"id":        200,
+			"body":      "LGTM",
+			"user":      map[string]any{"login": "bot"},
+			"state":     "APPROVED",
+			"commit_id": "deadbeef1234",
+		})
+	}))
+	defer server.Close()
+
+	client := NewClient(server.URL, "test-token")
+	review, err := client.PostReview(context.Background(), "owner", "repo", 5, "APPROVED", "LGTM", "deadbeef1234", nil)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if gotPayload.CommitID != "deadbeef1234" {
+		t.Errorf("sent commit_id = %q, want %q", gotPayload.CommitID, "deadbeef1234")
+	}
+	if review.CommitID != "deadbeef1234" {
+		t.Errorf("response commit_id = %q, want %q", review.CommitID, "deadbeef1234")
+	}
+}
+
+func TestPostReview_EmptyCommitID_OmittedFromPayload(t *testing.T) {
+	var gotRaw map[string]any
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		json.NewDecoder(r.Body).Decode(&gotRaw)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(200)
+		json.NewEncoder(w).Encode(map[string]any{
+			"id":   201,
+			"body": "ok",
+			"user": map[string]any{"login": "bot"},
+		})
+	}))
+	defer server.Close()
+
+	client := NewClient(server.URL, "test-token")
+	_, err := client.PostReview(context.Background(), "owner", "repo", 5, "COMMENT", "ok", "", nil)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if _, exists := gotRaw["commit_id"]; exists {
+		t.Errorf("commit_id should be omitted when empty, but was present: %v", gotRaw["commit_id"])
+	}
+}