fix(armory): report empty package lock status

[cwilson613]

Summary

Fixes #20.

nex lock status now treats an absent Armory package lock as a valid empty state instead of surfacing an os error 2 read failure.

Output with no lock:

Armory package lock: absent
roots: 0
materialized packages: 0

lock status is also handled before config resolution, so it works on fresh systems that do not have a Nex repo/config yet.

Tests

nix shell nixpkgs#cargo nixpkgs#rustc nixpkgs#rustfmt -c cargo fmt
nix shell nixpkgs#cargo nixpkgs#rustc -c cargo test lock_status -- --nocapture
nix shell nixpkgs#cargo nixpkgs#rustc -c cargo test

Manual fresh-HOME check:

tmp=$(mktemp -d)
HOME=$tmp nix shell nixpkgs#cargo nixpkgs#rustc -c cargo run -- lock status
rm -rf $tmp

Result:

Armory package lock: absent
roots: 0
materialized packages: 0

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