Add pkg-inventory.sh: report what a machine has outgrown its manifests

install.sh installs desired − installed from packages/*.pkgs, which guarantees a fresh box reaches
the declared state — and nothing anywhere checked that the declared state still describes the
machines. Measured against the Calamares install day, blankpad carries 41 hand-installed packages
and packages/blankpad.pkgs declared one line.

pkg-inventory.sh walks the other direction: installed − declared, with the pacman log as the
baseline so installer furniture is never reported as a hand install. It splits additions by
whether a sync db can serve them (Chaotic-AUR counts, so only true AUR routes to aur.pkgs),
excludes orphans from --emit, reports flatpak apps without pretending install.sh installs them,
and exits 3 rather than guessing when the sync databases are unreadable.

16 black-box tests over fixtures and a stubbed pacman. Two of them exist because the first
implementation was wrong in ways a small fixture cannot show: sort|head under pipefail dies of
SIGPIPE on a real 1000-line install-day log (awk-exit closes the pipe exactly as head does), and
tr without a trailing echo glues the next report label onto the last folded name.

Docs: omp-docs/tools/packages.md now owns the package tier — install.sh was never documented.

Refs #51

omp-docs/tools/index.md

@@ -19,6 +19,7 @@ Configuration guides and reference material for installed software. Each tool ha
 * [neovim](neovim.md) — Two editors in one binary: vanilla `nvim` for quick edits and the `lazyvim` NVIM_APPNAME profile, with the base16 wallpaper-palette hook and fresh-box install.
 * [niri](niri/index.md) — Scrollable-tiling Wayland compositor — configuration, keybindings, and architecture for the workstation.
 * [noctalia-shell](noctalia-shell.md) — Native desktop environment for Niri — launcher, control center, settings, lock screen, session menu, hotkey overlay, media controls, and system management.
+* [packages](packages.md) — `packages/*.pkgs` desired state and the two tools that keep a machine and its manifest agreeing: `install.sh` installs the delta, `pkg-inventory.sh` reports what the machine has outgrown.
 * [tmux](tmux.md) — Terminal multiplexer configuration, extended-keys setup, keyboard input best practices, and reload/restart procedures.
 
 ## Adding a New Tool