profile: stop shadowing exit

`exit` was a function that ran `tmux detach` instead of exiting, whenever a
shell was interactive and inside tmux. What it bought was a keystroke: tmux
already binds detach to prefix-d, which this config does not touch.

What it cost was an afternoon. Claude Code captures shell functions into a
snapshot and sources it in every tool call, so agent shells inheriting $TMUX
ran the function too: terminal windows closed on their own for weeks before
anyone connected it to `exit`, and a poll loop that called `exit 0` detached a
client every minute instead of stopping, because the function detached and then
returned. The call-time guard added on 2026-07-16 fixed that case; it did not
make shadowing a POSIX builtin a good idea.

Nothing in either repo calls it and nothing documents the behaviour.

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