Move the command tree into internal/cli (closes #8)

cmd/simplexcalc/ now holds only main.go: it passes the version the
linker sets to cli.Main and exits with the status that returns. root.go
and run.go move to internal/cli with their comments, changed only to
take the version as a parameter. Main reads the process's arguments and
output; execute takes them as parameters so the new tests can check the
version line and the non-zero status of an unknown command. Commands,
flags, output, log lines and exit status are unchanged, and the
-X main.version flag in the Dockerfile still applies.

Model: opus-5-5

docs/TODO.md

@@ -27,6 +27,9 @@ with no deprecation warning.
 
 # Completed Steps
 
+- 2026-09-28 Moved the command tree and the `run` and `version` commands
+  from `cmd/simplexcalc/` into `internal/cli`; `cmd/simplexcalc/main.go`
+  is now a single call to `cli.Main`
 - 2026-09-26 Seeded from `go-template-repo` (renamed, its HTTP service
   removed) and built the bot: `internal/calc` for the arithmetic,
   `internal/simplex` for the chat client process and its WebSocket API,