contrib: add an uninstall script for the whole user install

contrib/install-desktop.sh uninstall only removes the GUI files. The new
contrib/uninstall.sh also removes signaltty's agent hooks and skills,
the systemd user service and the server/CLI binaries; --purge deletes
state and config. Agent worktrees and system packages are kept.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
6962d0badcSimone committed on 10/7/2026, 1:00:13 AMparent4979637

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