Add the ability to show package history

[anvacaru]

Adds a --history flag to kup list that shows the versions you have previously had installed for each package, reconstructed from the Nix profile generations.

• kup list --history — print the version timeline for every package that appears in the generation history.
• kup list <package> --history — print the timeline for a single package.

Each row shows the generation number, the date, and the version (git tag if known, otherwise the short commit; local checkout (<path>) for git+file:// builds). The version installed in the active generation is highlighted and marked with →. Consecutive generations with the same package version are collapsed, so the table shows version changes rather than every generation bump.

How it works

Every kup install/update/uninstall creates a new Nix profile generation, stored as a profile-<N>-link symlink whose target contains a manifest.json (the same format reload_packages already parses). The history is built by reading those manifests in order:

Changes

• src/kup/package.py: InstalledVersion dataclass, read_generation_manifests, GENERATION_LINK_RE.
• src/kup/__main__.py: profile_generations_dir, build_history, _mark_current, _enrich_tags, _format_version, list_history; --history argument on the list subparser and its dispatch. The duplicated GitHub auth block in the existing version listing is factored into _github_auth.
• src/kup/list-help.md, README.md: document the flag with an example.
• src/tests/unit/test_unit.py: unit tests for generation parsing (including garbage-collected generations and uri normalization), the collapsing and current-version marking in build_history, local-checkout entries, package filtering, version formatting, and profile symlink resolution. The tests run offline.