install: drop the cleanup of the 0.5 agent skill, v0.6.1

[rubas-agent]

Before

install.sh deletes any kagi, kagi-search, kagi-maps, or kagi-summarize dir in five agent skill dirs when the installed version is 0.5 or earlier:

case "$installed" in
0.[0-5].*)
  for dir in .agents/skills .claude/skills .codex/skills .gemini/antigravity-cli/skills .pi/agent/skills; do
    rm -rf "$HOME/$dir/kagi" ...

Problem

This removed the skill that releases up to 0.5.4 shipped. Every machine now runs 0.6.0 (asrock, expertbook, and epyc), so the code has no work left, and the smoke test kept testing it.

After

The installer only installs the three CLIs. The release smoke test checks an upgrade from 0.5.4 and a first install. The search skill in rubas/dotfiles covers these CLIs (rubas/dotfiles#581).

The new install.sh, run against the v0.6.0 archive in a scratch home with a fake 0.5.4:

kagi: 0.5.4 -> 0.6.0
installed kagi 0.6.0: kagi-search, kagi-maps, and kagi-summarize in .../home/.local/bin
$ ls ~/.agents/skills
kagi

Details

• Version 0.6.1 in Cargo.toml, Cargo.lock, flake.nix, and the README example, so the merge releases it.
• Tests: the scratch run above, sh -n, shellcheck, dprint check.