feat(vcs): add CommitID to ReviewRequest (#115)

Add CommitID string field to vcs.ReviewRequest so both platform
adapters can thread the commit anchor through the abstraction layer.

Changes:
- vcs/types.go: Add CommitID field with json tag commit_id,omitempty
- gitea/client.go: Re-add commitID parameter to PostReview (was
  removed during PR #112 refactoring)
- gitea/adapter.go: Forward req.CommitID to underlying client
- github/review.go: Use req.CommitID as primary anchor, fall back to
  comment-derived CommitID when empty, reject on conflict
- cmd/review-bot/main.go: Set ReviewRequest.CommitID = evaluatedSHA

Fixes #115

gitea/adapter_test.go

@@ -214,6 +214,40 @@ func TestAdapter_PostReview_EventTranslation(t *testing.T) {
 	}
 }
 
+
+func TestAdapter_PostReview_CommitID(t *testing.T) {
+	var gotCommitID string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		var payload struct {
+			CommitID string `json:"commit_id"`
+		}
+		json.NewDecoder(r.Body).Decode(&payload)
+		gotCommitID = payload.CommitID
+		json.NewEncoder(w).Encode(map[string]any{
+			"id":        1,
+			"body":      "test",
+			"user":      map[string]any{"login": "bot"},
+			"commit_id": payload.CommitID,
+		})
+	}))
+	defer server.Close()
+
+	client := gitea.NewClient(server.URL, "token")
+	adapter := gitea.NewAdapter(client)
+
+	_, err := adapter.PostReview(context.Background(), "owner", "repo", 1, vcs.ReviewRequest{
+		Body:     "test",
+		Event:    vcs.ReviewEventApprove,
+		CommitID: "sha256abc",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if gotCommitID != "sha256abc" {
+		t.Errorf("expected commit_id %q forwarded to client, got %q", "sha256abc", gotCommitID)
+	}
+}
 func TestAdapter_PostReview_WithComments_PositionTranslation(t *testing.T) {
 	diff := `diff --git a/main.go b/main.go
 --- a/main.go