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

AGENTS.md

@@ -1,6 +1,6 @@
 ---
 title: Agent Guidance
-last_modified: 2026-09-26
+last_modified: 2026-09-28
 ---
 
 This file is the single source of guidance for any automated agent
@@ -118,9 +118,10 @@ Do not weaken them.
 ## Layout
 
 ```
-cmd/simplexcalc/    cobra command tree; main(), run and version
+cmd/simplexcalc/    main(), a single call into internal/cli
 internal/bot/       startup, address setup, and the reply to a message
 internal/calc/      the arithmetic: go/parser and go/constant
+internal/cli/       cobra command tree: run and version
 internal/config/    viper-backed configuration; the abort-on-garbage rule
 internal/logger/    log/slog, JSON always
 internal/simplex/   the simplex-chat child process and its WebSocket API