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>

lib/src/daemon/dispatcher.dart

@@ -1,4 +1,5 @@
 import 'package:clide/clide.dart' show clideVersion;
+import 'package:clide/src/ipc/command_schema.dart';
 import 'package:clide/src/ipc/envelope.dart';
 import 'package:clide/src/ipc/schema_v1.dart';
 
@@ -12,13 +13,26 @@ class DaemonDispatcher {
 
   final Map<String, CommandHandler> _handlers = {};
 
-  void register(String cmd, CommandHandler handler) {
+  /// Per-command argument schemas (T-120 / D-74). Populated alongside
+  /// handlers by registrants. A command with no entry dispatches
+  /// unvalidated — schema adoption is opt-in per command.
+  final Map<String, CommandSchema> _schemas = {};
+
+  /// Register [handler] for [cmd]. Pass [schema] to have the dispatcher
+  /// normalise + validate `req.args` before the handler runs (D-74).
+  void register(String cmd, CommandHandler handler, {CommandSchema? schema}) {
     _handlers[cmd] = handler;
+    if (schema != null) {
+      _schemas[cmd] = schema;
+    } else {
+      _schemas.remove(cmd);
+    }
   }
 
   /// Remove all registered handlers except ping/version.
   void clear() {
     _handlers.removeWhere((k, _) => k != 'ping' && k != 'version');
+    _schemas.removeWhere((k, _) => k != 'ping' && k != 'version');
   }
 
   bool get isEmpty => _handlers.length <= 2; // only ping + version
@@ -36,7 +50,13 @@ class DaemonDispatcher {
         ),
       );
     }
-    return h(req);
+    final schema = _schemas[req.cmd];
+    if (schema == null) return h(req);
+    final result = schema.validate(schema.normalize(req.args));
+    if (!result.isOk) {
+      return schemaError(req.id, result.error!);
+    }
+    return h(IpcRequest(id: req.id, cmd: req.cmd, args: result.values!));
   }
 
   Future<IpcResponse> _ping(IpcRequest req) async => IpcResponse.ok(