netwatch-server refuses arguments it does not understand (closes #179)

cli.Main starts the server only when given no arguments. check-cidr
CIDR and prepare-data-dir DATA_DIR run as before. Any other command
line, such as a subcommand with no value or two values, or a misspelt
subcommand, gets a one-line usage message naming both forms on stderr
and exit code 2. It used to start the server, which then held the
port.

Model: opus-5-5

TODO.md

@@ -22,6 +22,15 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-19: `netwatch-server` starts the server only when given no arguments
+  ([#179](https://git.eeqj.de/sneak/netwatch/issues/179)). `check-cidr CIDR` and
+  `prepare-data-dir DATA_DIR` behave as before; any other arguments, such as a
+  subcommand with no value or two, or a misspelt one, print a one-line usage
+  message naming both forms to stderr and exit 2. They used to start the server.
+  Unit tests call `cli.Main` with no arguments and check that it starts the
+  server, and with each subcommand given no value and two values and with a
+  misspelt subcommand, and check the exit code, the message and that the server
+  does not start
 - 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