feat(plugin): surface loaded plugin commands in command-line autocomplete

Autocomplete suggestions came only from all_registered_command_names() —
the compile-time inventory registry returning &'static str — so
runtime-loaded plugin commands could never appear. Typing a plugin's
command prefix (e.g. `LS_`) showed nothing even with the plugin loaded
and its ribbon tab active; dispatch worked, only discovery was blind.

Collect each enabled plugin's ribbon ToolDef command ids plus its
manifest command_prefixes into a dynamic candidate pool
(OwnedRibbonGroup::command_ids → PluginManager::command_names →
plugin_command_names), refreshed in rebuild_ribbon_modules on startup
load, settings reload, and every enable/disable toggle. ranked_matches
now merges this pool with the static registry and returns owned strings.

A plugin.toml `commands = [...]` list for sub-verbs with no ribbon button
(e.g. LS_AUTOLABEL) stays a follow-up — it's a plugin-API surface change.

Closes #272

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/update/command.rs

@@ -194,8 +194,7 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                 // rather than the partial text actually in the buffer.
                 let i_tab = self.active_tab;
                 if self.tabs[i_tab].active_cmd.is_none() {
-                    if let Some(picked) = self.command_line.selected_suggestion() {
-                        let cmd = picked.to_string();
+                    if let Some(cmd) = self.command_line.selected_suggestion() {
                         self.command_line.input.clear();
                         self.command_line.autocomplete_cursor = None;
                         return self.dispatch_command(&cmd);