Add initial distributor CLI skeleton

docs/cli.md

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+# Distributor CLI
+
+## Shortest useful command
+
+```sh
+go run ./cmd/distributor --help
+```
+
+This prints the currently implemented command shell.
+
+## Command overview
+
+```sh
+distributor --help
+distributor version
+distributor run
+distributor validate
+distributor inspect
+```
+
+`version` prints the application name and version. The default development version is `dev`; release builds may replace it at build time.
+
+`run`, `validate`, and `inspect` are command placeholders. They intentionally fail with a clear `not implemented` error until the corresponding application behavior exists.
+
+## Flag reference
+
+The root command supports:
+
+- `--help`, `-h`: print root help.
+
+Each implemented subcommand supports:
+
+- `--help`, `-h`: print command-specific help.