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

.gitignore

@@ -1,8 +1,5 @@
 # built binaries (repo root only)
 /chlog
-/chcat
-/chtail
-/chgrep
 # cross-compiled release artifacts (e.g. chlog-linux-amd64)
 /chlog-*
 dist/