[FEATURE]: Add a /permissions command to set specific permissions for the agent

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

Instead of it being ask or auto-approve for all tools, I would like a settings section accessed from slash command like /permissions or similar, where you are able to choose specific permissions for tools. Things like is MCP use allowed, web search, safe commands and dangerous commands are also able to be set. Plus, with things like read and write, as in is the agent auto approved to read your project files, is it allowed to read all of your files, same with write. This would help a lot if I'm deep in a session and I don't want to use confusing commands and do something I don't want too, or have to edit my opencode.json.

Start by locating the slash-command entry point for /permissions and reviewing how permission settings are currently represented in opencode.json. Define how users choose MCP, web search, safe and dangerous commands, and read/write scopes, with completion demonstrated by those settings changing approval behavior without manual config edits.