feat: add a permission-gated UI action registry and action slots for plugins

Moves the terminal toolbar's hardwired AI button into a slot the ssh-terminal
plugin offers and the ai plugin fills, gated on ai.services.use.

src/ui/locales/en.json

@@ -5048,7 +5048,8 @@
     "terminalPanelPlaceholder": "Ask about this host...",
     "noProvidersConfigured": "No AI provider is configured yet. Add one in the AI assistant tab.",
     "runsInOpenTerminal": "Runs in your open terminal session.",
-    "runInTerminal": "Run"
+    "runInTerminal": "Run",
+    "terminalContextPrefill": "Here is my terminal output:\n\n{{output}}\n\n"
   },
   "macros": {
     "title": "Macros",