fix(github): escapePath returns error on dot-segments, fix Description semantics

- escapePath now returns an error when paths contain dot-segments
  (".", "..") instead of silently rewriting them. This prevents
  subtle API misses where callers pass "foo/../bar" expecting to
  hit "bar" but the old code produced "foo/bar".
- Uses path.Clean for canonical form after validation.
- CommitStatus.Description for check runs is now empty string
  instead of the raw conclusion enum. The conclusion is already
  captured in the Status field via mapCheckRunStatus; storing it
  again in Description was semantically inconsistent with commit
  statuses where Description carries a human-readable narrative.
- Removed unused derefString helper.
- Added tests for escapePath valid paths, dot-segment rejection,
  and GetFileContentAtRef dot-segment error propagation.

github/files_test.go

@@ -0,0 +1,79 @@
+package github
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestEscapePath_ValidPaths(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		path string
+		want string
+	}{
+		{"simple file", "file.go", "file.go"},
+		{"nested path", "path/to/file.go", "path/to/file.go"},
+		{"special chars", "path/to/my file.go", "path/to/my%20file.go"},
+		{"leading slash stripped", "/path/to/file.go", "path/to/file.go"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got, err := escapePath(tt.path)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+			if got != tt.want {
+				t.Errorf("escapePath(%q) = %q, want %q", tt.path, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestEscapePath_DotSegments(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		path string
+	}{
+		{"single dot", "./file.go"},
+		{"double dot", "../file.go"},
+		{"dot in middle", "path/./file.go"},
+		{"parent traversal", "path/../file.go"},
+		{"only dots", ".."},
+		{"nested parent traversal", "a/b/../../c"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			_, err := escapePath(tt.path)
+			if err == nil {
+				t.Fatalf("expected error for path %q, got nil", tt.path)
+			}
+			if !strings.Contains(err.Error(), "dot-segment") {
+				t.Errorf("expected error about dot-segment, got: %v", err)
+			}
+		})
+	}
+}
+
+func TestGetFileContentAtRef_DotSegmentError(t *testing.T) {
+	// Server should never be called — the error is caught before the request.
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatal("server should not have been called")
+	}))
+	defer srv.Close()
+
+	c := NewClient(srv.URL, "token")
+	_, err := c.GetFileContentAtRef(context.Background(), "owner", "repo", "foo/../bar.go", "main")
+	if err == nil {
+		t.Fatal("expected error for path with dot-segments")
+	}
+	if !strings.Contains(err.Error(), "invalid file path") {
+		t.Errorf("expected 'invalid file path' error, got: %v", err)
+	}
+}