Closes AI-432

The turn-end action buttons in the agent panel (scroll to top, scroll to
most recent prompt, open as markdown, thumbs up/down, share/sync) are
now exposed as local slash commands in the message editor instead of
buttons rendered at the end of each turn. The actions themselves are
unchanged — this is just a different way to access them.

Release Notes:

- Changed some of the agent panel's turn-end action buttons into slash
commands in the message editor.

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Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>