[FEATURE]: expose the plugin manager as a /plugins slash command

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

The TUI plugin manager already exists and works well, but it has no slash command, so it is effectively undiscoverable.

packages/tui/src/feature-plugins/system/plugins.tsx registers two palette commands:

• plugins.list (title "Plugins") - lists built-in and external plugins, space toggles enable/disable
• plugins.install (title "Install plugin") - npm install prompt with local/global scope

Neither declares a slashName. Both default keybinds are none (packages/tui/src/config/keybind.ts:226-227). So the only way to reach plugin lifecycle management is to open the command palette and scroll, or hand-configure a keybind.

Every comparable dialog in opencode already has a slash command: /mcps (mcp.list), /agents (agent.list), /models (model.list), /skills (prompt.skills), /themes, /status, /variants, /workspaces. Plugins are the outlier.

This is also the one piece of Claude Code parity people keep asking for. /plugin is where Claude Code users expect plugin lifecycle to live, and it comes up repeatedly:

• #23800 ("more user-friendly plugin management command ... something like ClaudeCode /plugin /plugins") - closed as duplicate of the marketplace megaissue #28696
• #7716 ("add /plugins commands to discover Plugins, MCPs, and Skills (like Claude Code)") - closed
• #44452 asks for plugin enable/disable management surfaces

Those all got folded into #28696, which is a large marketplace/registry effort. But the underlying management UI already shipped. Only the entry point is missing, and that is a two-line fix, independent of any marketplace work.

Proposal

Add slash names to the existing commands. No new UI, no new API, no behavior change.

• plugins.list -> /plugins, alias /plugin
• plugins.install -> /plugin-install

/plugins follows the existing plural convention (/mcps, /agents). /plugin as an alias gives Claude Code users the name they reach for. Neither collides with an existing slash name or alias.

Acceptance criteria

• /plugins and /plugin open the plugin manager dialog
• /plugin-install opens the install prompt
• Both appear in prompt slash autocomplete
• No change to existing keybinds, palette entries, or dialog behavior