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>

test/kernel/src/toolchain_paths_test.dart

@@ -10,11 +10,10 @@ void main() {
   group('ToolchainView.resolved', () {
     test('exposes the supplied paths verbatim', () {
       final v = ToolchainView.resolved(
-        const ResolvedPaths(git: '/opt/git', pql: '/opt/pql', tmux: '/opt/tmux', shell: '/usr/bin/zsh', gitEnv: {'GIT_EXEC_PATH': '/opt/git-core'}),
+        const ResolvedPaths(git: '/opt/git', pql: '/opt/pql', shell: '/usr/bin/zsh', gitEnv: {'GIT_EXEC_PATH': '/opt/git-core'}),
       );
       expect(v.git, '/opt/git');
       expect(v.pql, '/opt/pql');
-      expect(v.tmux, '/opt/tmux');
       expect(v.shell, '/usr/bin/zsh');
       expect(v.gitEnv, {'GIT_EXEC_PATH': '/opt/git-core'});
       expect(v.resolved, isTrue);
@@ -26,22 +25,21 @@ void main() {
       final v = ToolchainView.resolved(const ResolvedPaths());
       expect(v.git, 'git');
       expect(v.pql, 'pql');
-      expect(v.tmux, 'tmux');
       expect(v.shell, '/bin/bash');
       expect(v.gitEnv, isNull);
       expect(v.resolved, isTrue);
       expect(v.allOk, isFalse);
-      expect(v.missing, ['git', 'pql', 'tmux']);
+      expect(v.missing, ['git', 'pql']);
     });
 
     test('missing reports only the unresolved tools', () {
       final v = ToolchainView.resolved(
         const ResolvedPaths(
           git: '/opt/git',
-          // pql + tmux null → missing.
+          // pql null → missing.
         ),
       );
-      expect(v.missing, ['pql', 'tmux']);
+      expect(v.missing, ['pql']);
       expect(v.allOk, isFalse);
     });
   });

test/kernel/src/toolchain_test.dart

@@ -13,26 +13,17 @@ void main() {
       final t = Toolchain();
       expect(t.git, 'git');
       expect(t.pql, 'pql');
-      expect(t.tmux, 'tmux');
       expect(t.shell, '/bin/bash');
       expect(t.resolved, isFalse);
       expect(t.allOk, isFalse);
-      expect(t.missing, ['git', 'pql', 'tmux']);
+      expect(t.missing, ['git', 'pql']);
     });
 
     test('applyResolved with full paths flips resolved + allOk + clears missing', () {
       final t = Toolchain();
       var calls = 0;
       t.addListener(() => calls++);
-      t.applyResolved(
-        const ResolvedPaths(
-          git: '/usr/bin/git',
-          pql: '/usr/bin/pql',
-          tmux: '/usr/bin/tmux',
-          shell: '/bin/bash',
-          gitEnv: {'GIT_EXEC_PATH': '/usr/lib/git-core'},
-        ),
-      );
+      t.applyResolved(const ResolvedPaths(git: '/usr/bin/git', pql: '/usr/bin/pql', shell: '/bin/bash', gitEnv: {'GIT_EXEC_PATH': '/usr/lib/git-core'}));
       expect(t.resolved, isTrue);
       expect(t.allOk, isTrue);
       expect(t.missing, isEmpty);
@@ -46,7 +37,7 @@ void main() {
       t.applyResolved(const ResolvedPaths(pql: '/usr/bin/pql'));
       expect(t.resolved, isTrue);
       expect(t.allOk, isFalse);
-      expect(t.missing, ['git', 'tmux']);
+      expect(t.missing, ['git']);
     });
 
     test('waitForResolution completes immediately when already resolved', () async {