build(pkg): provision cs for native installs via a 'code-search' split package

The Docker path (bin/up) already builds and bundles cs, but the native
package path (bin/service install -> bin/build-pkg -> makepkg) did not, so
search_code would have no cs binary under a systemd/s6 install and fall back
to its 'cs not found' error.

- packaging/PKGBUILD: add 'code-search' to the split pkgname array; add go+git
  to makedepends; build the patched, statically-linked cs (pinned v3.1.0
  commit + docker/cs/luau-declarations.patch) in build(); install it to
  /usr/bin/cs via a dedicated package_code-search() (with the upstream MIT
  LICENSE). Pin kept in lockstep with the Dockerfiles.
- bin/install-pkg: install code-search alongside dispatch + dispatch-systemd
  by default (and under --all).

Verified: full  run produces code-search-0.0.1-1-x86_64.pkg.tar.zst
containing a working /usr/bin/cs (v3.1.0, static) with Luau declaration
detection; the dispatch package no longer contains cs. Tests (598) + biome
still pass.

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