metis: import desktop-env bundles (ADR-028 slice 3d)

[dannyfaris]

Summary

First commit that imports the slice-3 desktop bundles into a real host. Two-line addition to hosts/metis/default.nix:

• modules/core/nixos/bundles/desktop-env.nix → imports.
• home/core/nixos/bundles/desktop-env.nix → hostContext.extraHomeModules.

This is the integration test for the five prior slice-3 PRs:

	PR	State
3a — flake inputs	#42	Merged
3b — system bundle	#44	Merged
3c — home bundle	#55	Merged
3b.5 — niri cachix in nix.settings	#57	Merged
3b.5 CI half — extra_nix_config trust	#59	Merged
3d — this PR	this one	Open

CI now trusts niri.cachix.org (via #59 on main), so this PR's flake-check actually exercises the desktop-env closure with the cache as substituter — no source-build of niri, no crates.io 403 cascade.

Expected closure delta on metis

niri (from niri.cachix.org), greetd + tuigreet, Quickshell + Qt6 + matugen + DMS deps, xdg-desktop-portal-gnome, polkit-kde-agent-1, gnome-keyring, Inter, Noto Color Emoji, foot. No regression expected on mercury or nixos-vm — neither imports the bundles.

Local verification caveat

dbf is not in nix's trusted-users on the operator's machine, so runtime substituter additions are rejected with warning: ignoring untrusted substituter. Local nix build .#nixosConfigurations.metis... would source-build niri. The integration test fires in CI where the GH Actions runner is trusted and the workflow declares extra-substituters.

No nh os switch here — that's slice 5.

Peer review

Reviewed against 8 checkpoints:

1. Bundle placement reasonable (system import next to remote-access; home import grouped with shared bundles).
2. No other host imports the bundle; no auto-discovery anywhere in the flake.
3. Closure delta list verified against the four constituent slice-3 modules.
4. nix flake check --no-build succeeds independent of cache trust (eval doesn't require fetching).
5. CI matrix correctly scopes metis to x86_64-linux only — no aarch64 poisoning.
6. CI ordering safe (now moot since ci: trust niri.cachix.org substituter in flake-check workflow (ADR-028) #59 is on main).
7. Merge-order safety (also now moot).
8. HM module order is cosmetic (option merge order-independent), no Stylix font conflicts, all pre-commit hooks no-op.

Test plan

• Parse + treefmt clean.
• nix flake check --no-build succeeds (eval).
• Pre-commit hooks pass (deadnix, statix).
• CI matrix green on x86_64-linux — the real integration test. niri pulled from niri.cachix.org; DMS + Quickshell + Qt6 closure built; foot + greetd added.
• CI matrix green on aarch64-linux (no-op for this host).

Closes the ADR-028 slice 3 implementation. Slice 4 (DMS↔Stylix theme wiring) is tracked at #34; slice 5 (first activation on metis) at #35.

Refs: #33, ADR-028.

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