When `use_modifier_to_send` is turned on, holding `cmd`/`ctrl` is
necessary to send a message in the agent panel. Text threads already use
`cmd-enter` by default to submit a message, and it was done this way to
have the usual text editing bindings not taken over when writing a
prompt, sort of stimulating more thoughtful writing. While `enter` to
send is still somewhat a huge pattern in chat-like LLM UIs, it still
makes sense to allow this for the new agent panel... hence the existence
of this setting now!
Release Notes:
- agent: Added the `use_modifier_to_send` setting, which makes holding a
modifier (`cmd`/`ctrl`), together with `enter`, required to send a new
message.
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