feat(cli): clide claude account verbs — add/list/login/set/unset/remove (T-480 part 1)

The CLI half of the multi-account feature (epic T-476; D-6 parity). A new
`claude.account` dispatcher command multiplexes the six sub-verbs over an
injected, Flutter-free AccountStore port (runs under `dart test`):

- add <name> [--dir]   register (default ~/.claude-<name>); idempotent, clear
                       conflict error
- list                 {accounts, boundAccount (this workspace), detected}
- set <name>           bind this workspace (persists)
- unset                clear this workspace's binding
- remove <name> [--purge]  registry-remove; refuses while any workspace is
                       bound
- login <name>         (publishes the login action)

Registry reads/writes go through the user-scope SettingsStore; side-effects
that only the UI layer can do — respawn on set/unset, the `claude login`
terminal pane, and the --purge rm — are published on accountActionChannel for
the Claude extension to consume (that consumer is T-480 part 2). main.dart
adapts the real AccountRegistry to the port and registers the command alongside
image.show / status.

Adds SettingsStore.keysAt (binding enumeration) and AccountRegistry.boundName /
boundAccountNames. No changelog yet — set/unset don't auto-respawn until part 2,
so the feature isn't user-complete. Verb behaviour + payloads + the in-use
guard are unit-tested against a fake store.

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

lib/kernel/src/settings.dart

@@ -113,6 +113,15 @@ class SettingsStore extends ChangeNotifier {
     SettingsScope.ext => null,
   };
 
+  /// Every key currently stored in [layer] (no cross-layer merge) — for
+  /// prefix-scan consumers like the per-workspace account-binding enumerator
+  /// (T-480). [SettingsScope.ext] is a key class, not a layer → empty.
+  Iterable<String> keysAt(SettingsScope layer) => switch (layer) {
+    SettingsScope.app => _appValues.keys,
+    SettingsScope.project => _projectValues.keys,
+    SettingsScope.ext => const <String>[],
+  };
+
   /// The storage layer currently supplying [key]'s value (project overrides app
   /// for `ext.*`), or null when unset (Default). Honors the key's prefix.
   SettingsScope? effectiveLayer(String key) {