'completion' subcommand is missing from 'toolbox --help'

**Describe the bug**

`toolbox completion {bash,fish,zsh}` is a real, working subcommand (generates
shell completion scripts via Cobra), but it does not show up anywhere in
`toolbox --help` / `toolbox -h`, nor in the `toolbox(1)` man page or the
README. It's marked `Hidden: true` in `src/cmd/completion.go`, which removes
it from the command listing entirely.

This makes the command effectively undiscoverable for anyone who isn't
already reading the source, e.g. users who build from source or run on a
distro that doesn't already ship pre-generated completions and want to
regenerate them manually.

**Steps how to reproduce the behaviour**
1. `toolbox --help`
2. Look for a 'completion' entry in the command list — there isn't one.
3. `toolbox completion bash` still works fine if you happen to already know
the exact subcommand and argument to type.

**Expected behaviour**

Either:
- `completion` shows up in `toolbox --help` / the command listing, or
- if it's intentionally hidden from the interactive command list (e.g.
because it's meant to be invoked by packaging/build tooling rather than by
end users directly), it should at least be documented somewhere
user-facing (`toolbox(1)` man page or README) so people know it exists.

**Actual behaviour**

`completion` is invisible in `--help`, the man page, and the README — the
only way to discover it is by reading `src/cmd/completion.go`.

**Output of `toolbox --version` (v0.0.90+)**
```
toolbox version 0.3
```

**Toolbx package info (`rpm -q toolbox`)**
```
toolbox-0.3-4.fc44.x86_64
```

**Output of `podman version`**
```
Client: Podman Engine
Version: 5.8.4
API Version: 5.8.4
Go Version: go1.26.4-X:nodwarf5
Git Commit: 5431df23c742e5edea35bef34eed696f4db0106b
```

**Podman package info (`rpm -q podman`)**
```
podman-5.8.4-1.fc44.x86_64
```

**Info about your OS**
Fedora Linux 44 (Workstation Edition)

**Additional context**

Looked through the Git history: `src/cmd/completion.go` was deliberately
marked `Hidden: true` (commit 4469774, "cmd/completion: Add prefix to
command to hide it better"), so this may be intentional design rather than
an oversight. Filing this so the discoverability gap is tracked either way —
happy to send a PR to add a mention of `toolbox completion` to the man page
or README if that's the preferred fix instead of un-hiding the subcommand.

Start in src/cmd/completion.go and inspect how the Hidden setting affects Cobra help output. Check the generated toolbox(1) man page and README for existing command documentation, then choose the project's preferred visibility or documentation approach. Done means users can discover toolbox completion through help or a user-facing document, with relevant checks passing.

Written by the indexing model from the issue text.