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>

test/builtin/claude/account_registry_test.dart

@@ -93,6 +93,23 @@ void main() {
       expect(reg.accountForWorkspace('/repo/a')?.name, 'personal');
       expect(reg.accountForWorkspace('/repo/b')?.name, 'work');
     });
+
+    test('boundName is the raw binding (survives account removal); boundAccountNames lists in-use names', () async {
+      await reg.registerAccount('personal', '/p');
+      await reg.registerAccount('work', '/w');
+      expect(reg.boundName('/repo/a'), isNull);
+      expect(reg.boundAccountNames(), isEmpty);
+
+      await reg.bindWorkspace('/repo/a', 'personal');
+      await reg.bindWorkspace('/repo/b', 'work');
+      expect(reg.boundName('/repo/a'), 'personal');
+      expect(reg.boundAccountNames(), {'personal', 'work'});
+
+      // Removing the account leaves the raw binding; only resolution degrades.
+      await reg.removeAccount('personal');
+      expect(reg.boundName('/repo/a'), 'personal');
+      expect(reg.accountForWorkspace('/repo/a'), isNull);
+    });
   });
 
   group('bootstrap probe', () {