fix(panes): avoid injecting prompt twice for literal agent commands

[danielroe]

I encountered an edge case where a prompt was being sent twice.

with agent: pi and panes: [{ command: pi, focus: true }], workmux add dup-probe -b -P PROMPT.md sends this to the pane:

sh -c ' pi "$(cat .workmux/PROMPT-dup-probe.md)" "$(cat .workmux/PROMPT-dup-probe.md)"'

the agent submits the first copy at startup and queues the second. using command: <agent> instead of the literal name doesn't reproduce (and is how I'm working around this locally for the moment).

I think this is because resolve_pane_command_with_config ends with resolved.command = resolved.render_command(), and then render_raw_command() reads self.command back. for a literal agent name, that string already contains the prompt.

this PR keeps the pre-prompt command in a private base_command field, but you may well (likely!) have another better solution 🙏

[raine]

Looks good, thanks!