remove tmux as a required tool (dead since D-77)

Nothing has spawned tmux since D-77 moved Claude session persistence to
`--resume`; Claude and terminal panes spawn `claude` or the shell directly.
But the toolchain still resolved tmux and listed it in `missing`, so on
mac/linux a box without tmux showed a spurious "tmux not found" warning in
the welcome view + status bar. The windows-support branch had special-cased
that away with a `!Platform.isWindows` guard — the tell that the requirement
was dead everywhere, not platform-specific.

Drop tmux from ResolvedPaths / ToolchainView / Toolchain (field, getter,
`missing`, PATH resolution) on every platform, removing the Windows guards
with it. Strip the testmode tmux probes and the comments / CLAUDE.md line
that claimed clide spawns tmux. (The dead ToolCheck class that also gated on
tmux was already deleted on main and dropped in the preceding merge.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/src/multitab_pane.dart

@@ -16,7 +16,7 @@ typedef MultitabEntryCallback<T> = void Function(MultitabEntry<T> entry);
 ///
 /// The widget is generic and domain-free: it never knows what's
 /// inside a tab. Hosts pick `T` and decide what add / close mean
-/// (e.g. spawning or killing a tmux session for the Claude pane).
+/// (e.g. spawning or killing a Claude session for the Claude pane).
 ///
 /// See `docs/design/multitab-pane.md` for the design rationale.
 class MultitabPane<T> extends StatelessWidget {