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/plugin/mod.rs

@@ -7,7 +7,7 @@ pub mod host;
 pub mod marketplace;
 pub mod registry;
 
-pub use registry::{all_ribbon_modules, ribbon_modules_enabled};
+pub use registry::{all_ribbon_modules, plugin_command_names, ribbon_modules_enabled};
 pub(crate) use registry::try_dispatch;
 
 /// Run a plugin entry point under a panic guard so a buggy external plugin