Add pacman package manager commands

[andrew]

Version: git-pkgs/managers current main at 241e284.

Trying to build an Arch package operation after registering the embedded definitions currently returns unknown manager: pacman:

_, err := tr.BuildCommand("pacman", "add", managers.CommandInput{
	Args: map[string]string{"package": "core/bash", "version": "5.3.3-1"},
})

Add a pacman definition for Arch's system package manager. Include definitions/pacman.yaml, captured version and command help under references/pacman, translator tests, and Docker coverage as described in the new manager contribution guide.

Pacman has no project manifest or lockfile, so this definition should be selected explicitly through DetectOptions.Manager. A PKGBUILD belongs to the makepkg build flow and should not trigger this installed-package manager.

Use the native pacman operations:

• Map add to pacman -S --needed --noconfirm PACKAGE and remove to pacman -R --noconfirm PACKAGE. The pacman(8) sync documentation covers repository-qualified targets such as testing/qt and constraints such as bash>=3.2.
• Map list to pacman -Q, outdated to pacman -Qu, and update to pacman -Syu --noconfirm. The query options define installed and upgrade filtering.
• Keep repository refresh and system upgrade in the same -Syu transaction. The sync options describe -u and recommend using -y with it. outdated should query the current sync database without silently running pacman -Sy first.

The generic install operation has no package set to pass to pacman -S, so leave it unsupported unless the public API gains a safe way to provide the desired packages. Pacman's -T operation checks named dependency constraints but does not return a dependency graph. -Ql returns all files owned by a package rather than one package directory. Do not advertise resolve or path for those commands.

Repository URLs are configured in pacman.conf, while a command can select a configured repository with the repository/package target syntax. The checked-in pacman.conf template provides an example configuration file for tests and documentation.

Reuse the per-command privilege metadata requested in #30. Generated commands should not prepend sudo; callers need to choose how system-changing operations are elevated. Use pacman's documented --noconfirm flag for non-interactive mutations.

Tests should cover repository-qualified names, exact and range versions, --needed, non-interactive add and remove commands, installed and outdated listings, the atomic -Syu update, and privilege metadata. Include cases proving that outdated does not refresh repositories and that unsupported install, resolve, and path capabilities are not declared.