docs: document the new config keys in the plan's close-out, item 123

Task 2's code review found that the README's sample config documents every
other key, including recently added ones, and has nothing for send_command
or the [compose] section. Without it those keys ship undiscoverable: a user
has no way to learn that sending exists at all.

That is a gap in the plan rather than a deviation by the task, since no task
claimed the README, so it becomes a step in the close-out where the rest of
the documentation is written.

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