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

lua/opencode/ui/completion/files.lua

@@ -144,7 +144,15 @@ local file_source = {
     local mention = require('opencode.ui.mention')
     mention.highlight_all_mentions(state.windows.input_buf)
 
-    context.add_file(item.data.full_path)
+    local mentioned_files = context.context.mentioned_files or {}
+    local is_already_mentioned = vim.tbl_contains(mentioned_files, item.data.full_path)
+
+    if is_already_mentioned then
+      context.remove_file(item.data.full_path)
+      mention.remove_mention(item.data.full_path)
+    else
+      context.add_file(item.data.full_path)
+    end
   end,
 }