Add slash commands for adding context into the assistant

[maxbrunsfeld]

Tasks

• remove old flaps and output when editing a slash command
• the completing a command name that takes args, insert a space to prepare for typing an arg
• always trigger completions when typing in a slash command
• don't show line numbers
• implement prompt command
• current-file command
• state gets corrupted on duplicate line up on a slash command
• exclude slash command source from completion request

Next steps:

• show output token count in flap trailer
• add /project command that matches project ambient context
• delete ambient context

Release Notes:

• N/A