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

`cmd/simplexcalc/` now holds only `main.go`, whose body is one call, `os.Exit(cli.Main(version))`, as the Go style guide requires. The command tree and the `run` and `version` commands moved unchanged into `internal/cli`, with tests for `version` and for an unknown command. Output streams, exit statuses and the `-X main.version` build flag are as before.

Disclosure: the comment in `execute` saying cobra has already printed the error is false, since the root command silences cobra's errors; it moved unchanged and is left for a follow-up.

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