feat(stash): register /stash in /help and autocomplete (#440 polish)

The slash command landed in 6fb87 but only via the dispatch
match arm — `/help` and the fuzzy autocomplete consult
`COMMANDS: &[CommandInfo]` to enumerate available commands, and
without a `CommandInfo` entry the new `/stash` was effectively
hidden from discovery.

Adds a `CommandInfo` row with `aliases: &["park"]`, a
`/stash [list|pop]` usage hint, and a new
`MessageId::CmdStashDescription` localized in en, ja, zh-Hans,
pt-BR. The description reminds users that Ctrl+S is the
matching push entry point — both surfaces should reinforce each
other in the help overlay.

No behavior change on the dispatch path; this is pure
discoverability.

crates/tui/src/commands/mod.rs

@@ -172,6 +172,12 @@ pub const COMMANDS: &[CommandInfo] = &[
         usage: "/queue [list|edit <n>|drop <n>|clear]",
         description_id: MessageId::CmdQueueDescription,
     },
+    CommandInfo {
+        name: "stash",
+        aliases: &["park"],
+        usage: "/stash [list|pop]",
+        description_id: MessageId::CmdStashDescription,
+    },
     CommandInfo {
         name: "subagents",
         aliases: &["agents"],

crates/tui/src/localization.rs

@@ -253,6 +253,7 @@ pub enum MessageId {
     CmdSettingsDescription,
     CmdSkillDescription,
     CmdSkillsDescription,
+    CmdStashDescription,
     CmdStatuslineDescription,
     CmdSubagentsDescription,
     CmdSwarmDescription,
@@ -437,6 +438,7 @@ pub const ALL_MESSAGE_IDS: &[MessageId] = &[
     MessageId::CmdSettingsDescription,
     MessageId::CmdSkillDescription,
     MessageId::CmdSkillsDescription,
+    MessageId::CmdStashDescription,
     MessageId::CmdStatuslineDescription,
     MessageId::CmdSubagentsDescription,
     MessageId::CmdSwarmDescription,
@@ -764,6 +766,9 @@ fn english(id: MessageId) -> &'static str {
         MessageId::CmdSkillsDescription => {
             "List local skills (or --remote to browse the curated registry)"
         }
+        MessageId::CmdStashDescription => {
+            "Park or restore a composer draft (Ctrl+S to push, /stash list/pop)"
+        }
         MessageId::CmdStatuslineDescription => "Configure which items appear in the footer",
         MessageId::CmdSubagentsDescription => "List sub-agent status",
         MessageId::CmdSwarmDescription => {
@@ -1037,6 +1042,9 @@ fn japanese(id: MessageId) -> Option<&'static str> {
         MessageId::CmdSkillsDescription => {
             "ローカルスキルを一覧表示（--remote で精選レジストリを参照）"
         }
+        MessageId::CmdStashDescription => {
+            "コンポーザーの下書きを退避／復元（Ctrl+S で退避、/stash list|pop）"
+        }
         MessageId::CmdStatuslineDescription => "フッターに表示する項目を設定",
         MessageId::CmdSubagentsDescription => "サブエージェントの状態を一覧表示",
         MessageId::CmdSwarmDescription => {
@@ -1286,6 +1294,7 @@ fn chinese_simplified(id: MessageId) -> Option<&'static str> {
         MessageId::CmdSettingsDescription => "显示持久化设置",
         MessageId::CmdSkillDescription => "激活技能，或安装/更新/卸载/信任社区技能",
         MessageId::CmdSkillsDescription => "列出本地技能（或使用 --remote 浏览精选注册表）",
+        MessageId::CmdStashDescription => "暂存或恢复输入草稿（Ctrl+S 暂存，/stash list|pop）",
         MessageId::CmdStatuslineDescription => "配置底栏要显示哪些条目",
         MessageId::CmdSubagentsDescription => "列出子代理状态",
         MessageId::CmdSwarmDescription => {
@@ -1545,6 +1554,9 @@ fn portuguese_brazil(id: MessageId) -> Option<&'static str> {
         MessageId::CmdSkillsDescription => {
             "Listar skills locais (ou --remote para navegar pelo registro curado)"
         }
+        MessageId::CmdStashDescription => {
+            "Estacionar ou restaurar rascunho do compositor (Ctrl+S estaciona, /stash list|pop)"
+        }
         MessageId::CmdStatuslineDescription => "Configurar quais itens aparecem no rodapé",
         MessageId::CmdSubagentsDescription => "Listar o status dos sub-agentes",
         MessageId::CmdSwarmDescription => {