The TUI had no way to toggle plugins — `/plugins` only printed a static
list, and the classic `hermes plugins` picker is curses-based and can't
run inside the Ink UI. Users had to drop to a separate shell and run
`hermes plugins enable/disable`.
Add a PluginsHub overlay modeled on the existing SkillsHub:
- New gateway RPC `plugins.manage` (list + toggle) backed by the same
disk-discovery + dashboard_set_agent_plugin_enabled primitives the CLI
and dashboard already use, so all three surfaces agree on state. The
toggle path also wires the plugin's toolset into platform_toolsets.
- `/plugins` with no arg opens the hub; any subcommand still falls
through to the text slash worker for CLI parity.
- pluginsHub overlay state threaded through overlayStore / interfaces /
useInputHandlers (Esc closes) / appOverlays (renders the FloatBox);
preserved across turn teardown like other user-toggled overlays.
- Hub UI: arrow/number select, Enter/Space toggles live, Tab switches
user-only vs all (bundled) scope, shows ✓/✗/○ activation glyphs.
plugins.manage added to _LONG_HANDLERS (disk + config I/O).
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