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/src/pty/env.dart

@@ -41,7 +41,7 @@ const Map<String, String> clidePtyEnvDefaults = {
   'COLORTERM': 'truecolor',
   // Encourages 24-bit emission from tooling that checks this:
   'CLICOLOR_FORCE': '1',
-  // tmux inherits these when clide spawns tmux; safe to propagate.
+  // UTF-8 locale for the child and anything it spawns; safe to propagate.
   'LANG': 'en_US.UTF-8',
   'LC_ALL': 'en_US.UTF-8',
 };

lib/src/pty/pty.dart

@@ -1,7 +1,7 @@
 /// PTY subsystem — spawn child processes under a PTY and expose their
 /// output as a byte stream. POSIX uses posix_openpt() + posix_spawn();
 /// Windows uses ConPTY. Desktop IDE's pane model (terminal / Claude /
-/// future tmux wrappers) rides on this.
+/// future PTY-backed panes) rides on this.
 library;
 
 export 'env.dart' show clidePtyEnvDefaults, mergePtyEnv;