feat: add plugin system with sandboxed runtime

Plugin discovery from ~/.config/bterminal/plugins/ with plugin.json
manifest. Sandboxed new Function() execution, permission-gated API
(palette, btmsg:read, bttask:read, events). Plugin store + SettingsTab.

v2/src-tauri/src/commands/plugins.rs

@@ -0,0 +1,20 @@
+// Plugin discovery and file access commands
+
+use crate::AppState;
+use crate::plugins;
+
+#[tauri::command]
+pub fn plugins_discover(state: tauri::State<'_, AppState>) -> Vec<plugins::PluginMeta> {
+    let plugins_dir = state.app_config.plugins_dir();
+    plugins::discover_plugins(&plugins_dir)
+}
+
+#[tauri::command]
+pub fn plugin_read_file(
+    state: tauri::State<'_, AppState>,
+    plugin_id: String,
+    filename: String,
+) -> Result<String, String> {
+    let plugins_dir = state.app_config.plugins_dir();
+    plugins::read_plugin_file(&plugins_dir, &plugin_id, &filename)
+}