Make build-rpm.sh executable and test completion generation

The v0.1.0 tag pipeline failed at the package step: build-rpm.sh was
committed mode 100644, so CI's ./scripts/build-rpm.sh invocation died
with permission denied before any completions were generated.

- Restore the executable bit on scripts/build-rpm.sh (mode 100755,
  matching node-lookup)
- Add TestCompletionAllEntrypointsAndShells covering bash/zsh/fish
  completion generation through the argv[0] dispatch for all four
  entrypoints (chlog/chcat/chtail/chgrep)

main_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"os"
 	"strings"
 	"testing"
 	"time"
@@ -82,6 +83,32 @@ func TestGrepGuardAllowsShortWindow(t *testing.T) {
 	}
 }
 
+func TestCompletionAllEntrypointsAndShells(t *testing.T) {
+	// scripts/build-rpm.sh runs "<name> completion <shell>" for every
+	// entrypoint; guard that each argv[0]-dispatched command tree exposes a
+	// working completion subcommand for all three packaged shells.
+	for _, name := range []string{"chlog", "chcat", "chtail", "chgrep"} {
+		for _, shell := range []string{"bash", "zsh", "fish"} {
+			t.Run(name+"/"+shell, func(t *testing.T) {
+				orig := os.Args
+				os.Args = []string{name}
+				defer func() { os.Args = orig }()
+				cmd := entrypoint()
+				var out strings.Builder
+				cmd.SetOut(&out)
+				cmd.SetErr(&out)
+				cmd.SetArgs([]string{"completion", shell})
+				if err := cmd.Execute(); err != nil {
+					t.Fatalf("%s completion %s: %v", name, shell, err)
+				}
+				if !strings.Contains(out.String(), name) {
+					t.Fatalf("%s completion %s output does not mention %q", name, shell, name)
+				}
+			})
+		}
+	}
+}
+
 func TestEntrypointDispatch(t *testing.T) {
 	root := newRootCmd()
 	names := map[string]bool{}