add in-app "Install clide command in PATH" affordance (T-212)

A command-palette / `clide` CLI verb (`clide.installCli`) copies the
bundled C client into ~/.local/bin, VS Code "Install code command" style,
so a user who runs the app without `make install` can still reach the CLI
from a shell. On launch the app detects when `clide` is missing from PATH
or resolves to a stale symlink into the Flutter GUI bundle (the dogfood
footgun: a bare `clide` launched a second app instead of querying the
socket) and notifies with a pointer to the command. `make build` now ships
the C client inside the app bundle so the affordance can self-install from
it; `make run` points it at the dev-tree client via CLIDE_CLI_BIN.

Detection and the copy live in the Flutter-free CliInstaller
(kernel/src/cli_install.dart) so they run under unit tests without a real
install.

Closes epic T-209 (ship the clide CLI on PATH).

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

lib/main.dart

@@ -5,6 +5,7 @@ import 'package:clide/test_app.dart';
 import 'package:clide/builtin/canvas/canvas.dart';
 import 'package:clide/builtin/claude/claude.dart';
 import 'package:clide/builtin/claude_control/claude_control.dart';
+import 'package:clide/builtin/cli_install/cli_install.dart';
 import 'package:clide/builtin/decisions/decisions.dart';
 import 'package:clide/builtin/default_layout/default_layout.dart';
 import 'package:clide/builtin/diff/diff.dart';
@@ -375,7 +376,8 @@ Future<void> main() async {
     ..register(SettingsUiExtension())
     ..register(ExtensionsUiExtension())
     ..register(KeybindingsUiExtension())
-    ..register(ClaudeControlExtension());
+    ..register(ClaudeControlExtension())
+    ..register(CliInstallExtension());
 
   await services.extensions.activateAll();