xtask: lint for xshell::Shell::new and xshell::cmd!

[justus-camp-microsoft]

#2743 added a macro to use an integrated xshell::Shell as part of rust flowey steps, but no way to enforce usage. This is problematic because future nodes could use xshell::cmd! and escape the nix-shell --pure wrapping that's coming.

This adds a lint as part of house-rules to catch usages and require an allow comment for any new future xshell::cmd! invocations. This required a refactor/removal of the PackageManager struct in install_dist_pkg to a more functional style that can keep the runtime around.

[Copilot]

Pull request overview

Adds an xtask fmt --house-rules lint to prevent Flowey code from directly using xshell::cmd! / xshell::Shell::new() (to avoid bypassing future nix-shell --pure wrapping), and updates/annotates existing exceptions accordingly.

Changes:

• Introduce a new house-rules check that flags bare xshell::cmd! / xshell::Shell::new() under flowey/ unless suppressed with // xtask-fmt allow-xshell <reason>.
• Wire the new lint into xtask house-rules with a --skip-flowey-xshell escape hatch.
• Refactor install_dist_pkg to use flowey::shell_cmd! / rt.sh instead of owning a standalone xshell::Shell, and add suppressions for remaining legitimate xshell uses.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
xtask/src/tasks/fmt/house_rules/flowey_xshell.rs	New house-rules lint that detects bare xshell usage in flowey/ and enforces justification-based suppressions.
xtask/src/tasks/fmt/house_rules.rs	Registers and runs the new lint; adds a CLI flag to skip it.
flowey/flowey_lib_common/src/run_cargo_doc.rs	Adds suppression annotation for an allowed xshell::cmd! usage in a utility context.
flowey/flowey_lib_common/src/install_dist_pkg.rs	Refactors package-install logic to use Flowey’s runtime shell (flowey::shell_cmd!) instead of creating a new xshell::Shell.
flowey/flowey_lib_common/src/_util/wslpath.rs	Annotates existing direct xshell usage with suppressions.
flowey/flowey_core/src/node.rs	Annotates core infrastructure xshell usage (runtime shell creation + macro wrapper) with suppressions.
flowey/flowey_cli/src/cli/regen.rs	Annotates CLI-specific xshell usage with suppressions.

[Copilot]

assert!(no_existing) can panic when the package query returns multiple lines that collapse to the same key after stripping the :arch suffix (e.g., multi-arch installs or glob matches from dpkg-query -W). Instead of asserting uniqueness, consider either keeping the full package identifier (including arch) or simply ignoring duplicates when inserting into the set.

Suggested change

	let no_existing = installed_packages.insert(package.to_owned());
	assert!(no_existing);
	// Ignore duplicates after normalizing package names (e.g., multi-arch variants).
	let _ = installed_packages.insert(package.to_owned());