Drop chcat/chtail/chgrep symlink entrypoints; ship chlog subcommands only

The RPM's /usr/bin/chcat conflicts with SELinux's
policycoreutils-python-utils on Fedora. Per Ben: remove the symlink
entrypoints entirely and ship only the chlog binary with cat/tail/grep
subcommands.

- Remove chcat/chtail/chgrep symlinks and their completions from
  nfpm.yaml, build-rpm.sh and the Makefile
- Remove the argv[0] dispatch in main.go; subcommands are unchanged
- Update the completion test to cover chlog only
- Update README usage to chlog cat|tail|grep

main_test.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"os"
 	"strings"
 	"testing"
 	"time"
@@ -41,7 +40,7 @@ func TestCommonFlagsFilterDefaults(t *testing.T) {
 }
 
 func TestGrepGuardBlocksWideUnfilteredSearch(t *testing.T) {
-	cmd := newGrepCmd("chgrep")
+	cmd := newGrepCmd()
 	cmd.SetArgs([]string{"--since", "24h", "needle"})
 	var out strings.Builder
 	cmd.SetOut(&out)
@@ -56,7 +55,7 @@ func TestGrepGuardAllowsFilteredSearch(t *testing.T) {
 	// A namespace filter disables the guard; the query then fails on the
 	// unreachable server rather than the guard, proving the guard passed.
 	t.Setenv("CH_URL", "http://127.0.0.1:1")
-	cmd := newGrepCmd("chgrep")
+	cmd := newGrepCmd()
 	cmd.SetArgs([]string{"--since", "24h", "--namespace", "logging", "needle"})
 	var out strings.Builder
 	cmd.SetOut(&out)
@@ -72,7 +71,7 @@ func TestGrepGuardAllowsFilteredSearch(t *testing.T) {
 
 func TestGrepGuardAllowsShortWindow(t *testing.T) {
 	t.Setenv("CH_URL", "http://127.0.0.1:1")
-	cmd := newGrepCmd("chgrep")
+	cmd := newGrepCmd()
 	cmd.SetArgs([]string{"--since", "1h", "needle"})
 	err := cmd.Execute()
 	if err == nil {
@@ -83,33 +82,27 @@ 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 TestCompletionAllShells(t *testing.T) {
+	// scripts/build-rpm.sh runs "chlog completion <shell>"; guard that a
+	// working completion subcommand exists for all three packaged shells.
+	for _, shell := range []string{"bash", "zsh", "fish"} {
+		t.Run(shell, func(t *testing.T) {
+			cmd := newRootCmd()
+			var out strings.Builder
+			cmd.SetOut(&out)
+			cmd.SetErr(&out)
+			cmd.SetArgs([]string{"completion", shell})
+			if err := cmd.Execute(); err != nil {
+				t.Fatalf("chlog completion %s: %v", shell, err)
+			}
+			if !strings.Contains(out.String(), "chlog") {
+				t.Fatalf("chlog completion %s output does not mention chlog", shell)
+			}
+		})
 	}
 }
 
-func TestEntrypointDispatch(t *testing.T) {
+func TestRootSubcommands(t *testing.T) {
 	root := newRootCmd()
 	names := map[string]bool{}
 	for _, c := range root.Commands() {