CLI tree: surface 'mock scene' as a direct sibling of 'mock scenario'

[ShortArrow]

Context

Today, the scene-management verbs live as a sub-tree of
`mock scenario`:

```
ivicli mock scenario scene add --match ... --respond ...
ivicli mock scenario scene remove
```

This is not obvious from `ivicli mock --help`, which only
shows `scenario` and doesn't hint that scenes are nested two
levels deep. Users (and the PSU sample) keep typing the more
intuitive `ivicli mock scene add ...` and getting a generic
"command not recognized" error.

See `src/IviCli.Cli/Commands/MockScenarioCommand.cs:25`:

```csharp
command.Subcommands.Add(MockSceneCommand.Build(services));
```

Proposal

Surface `scene` as a direct sibling of `scenario` under
`mock` (so both `ivicli mock scene add ...` and
`ivicli mock scenario scene add ...` work), or replace the
nested form with the flat form entirely.

Recommended: flat form only, with the nested form removed
in 0.2.x and a one-line deprecation note in the CLI help for
the bridge release.

Acceptance

• `ivicli mock --help` lists `scene` and `scenario` as
  peer subcommands.
• `ivicli mock scene add --match ... --respond ...`
  works.
• Docs (`docs/samples/psu/`, ADRs that reference the CLI tree)
  updated.