add typed IPC command-schema framework (T-120)

Per D-74: commands register an argument schema beside their handler
instead of hand-validating args inline. DaemonDispatcher accumulates a
cmd->schema registry and, before invoking a handler, normalises the
argv-translator shape ({positional, flags}) into named args, coerces
types, and checks per-arg constraints (charset/pattern, leading-dash
rejection, numeric range, list caps). Violations return userError so no
handler sees malformed input. Schema adoption is opt-in per command —
unschema'd commands dispatch unchanged.

panel.resize adopts a schema (dropping the _ResizeArgs hand-lift from
T-119); git.checkout and git.push gain schemas that reject leading-dash
refs at the dispatcher and, via positional ordering, fix the C-client
CLI path — `clide git checkout <branch>` now reaches the handler, where
the positional token previously never mapped to `branch`. The T-104
validateGitRef + count/path caps stay in place as defense-in-depth
because the git client is reachable directly from the UI, not only
through the dispatcher.

Co-Authored-By: Claude <noreply@anthropic.com>

test/daemon/git_commands_test.dart

@@ -252,6 +252,17 @@ void main() {
     expect(r.data['branch'], 'next');
   });
 
+  test('git.checkout via argv positional reaches the handler (D-74)', () async {
+    // `clide git checkout next` → {positional: ['next']}; the schema's
+    // positional ordering maps it to `branch` at the dispatcher.
+    await Process.run('git', ['branch', 'next'], workingDirectory: sandbox.path);
+    final r = await call('git.checkout', {
+      'positional': ['next'],
+    });
+    expect(r.ok, isTrue, reason: r.error?.message);
+    expect(r.data['branch'], 'next');
+  });
+
   test('git.checkout to an unknown branch surfaces a tool error', () async {
     final r = await call('git.checkout', {'branch': 'no-such-branch'});
     expect(r.ok, isFalse);

test/daemon/panel_commands_test.dart

@@ -55,7 +55,7 @@ void main() {
       final r = await call(const {'slot': 'sidebar', 'to': 'lots'});
       expect(r.ok, isFalse);
       expect(r.error!.kind, 'user_error');
-      expect(r.error!.message, contains('numeric'));
+      expect(r.error!.message, contains('number'));
     });
 
     test('rejects an unknown slot with not-found', () async {