main() is one call into internal/cli (closes #160)

cmd/netwatch-server/main.go parsed the command line, ran the check-cidr
and prepare-data-dir subcommands and built the fx application itself;
the Go style guide puts all of that in internal/ or pkg/. The new
package internal/cli does it, with the same arguments, messages and
exit codes. main() is now os.Exit(cli.Main(Appname, Version,
os.Args[1:], cli.RunServer)): Main takes the function that runs the
server, so its tests call it with one that fails them. Appname and
Version stay in main, where the build sets Version. Each subcommand
writes its error to a writer it is given, and prepare-data-dir also
takes the volume and the user's name, so its own tests give it a
temporary directory and the user they run as.

Model: opus-5-5

TODO.md

@@ -21,6 +21,18 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-19: `main()` in `cmd/netwatch-server/main.go` is one call into a new
+  package, `internal/cli`, as the Go style guide asks
+  ([#160](https://git.eeqj.de/sneak/netwatch/issues/160)). `internal/cli` runs
+  the `check-cidr` and `prepare-data-dir` subcommands, with the same arguments,
+  messages and exit codes, and otherwise the server, with the `fx` wiring.
+  `Appname` and `Version` stay in `main`, where the build sets `Version`, and
+  `main` gives `cli.Main` the function that runs the server, which the tests
+  replace with one that fails them. Unit tests call `cli.Main` with `check-cidr`
+  and a CIDR it takes and one it refuses, and with `prepare-data-dir` and a path
+  outside `/data`, and check the exit code. Others give each subcommand input it
+  takes and input it refuses, and check the exit code and what it writes to
+  stderr
 - 2026-10-19: report file writes no longer overlap
   ([#150](https://git.eeqj.de/sneak/netwatch/issues/150)), so the reports of
   failed writes are written in the order they arrived. Two writes that failed at