fix: improve context clearing in new sessions and file mentions

- Clear attachments when starting a new session with /new command
- Toggle file removal when selecting already-mentioned files with @ trigger
- Fixes issue where context persisted across new sessions
- Fixes issue where @ file selection only added files, never removed them

Author: aweis89

lua/opencode/core.lua

@@ -93,6 +93,7 @@ M.open = Promise.async(function(opts)
     if opts.new_session then
       state.active_session = nil
       state.last_sent_context = nil
+      context.unload_attachments()
 
       state.current_model = nil
       state.current_mode = nil