absentia v1.0.1

Patch release fixing a --version drift bug found minutes after v1.0.0 shipped, plus install-experience improvements.

Fixed

absentia --version reported the wrong version. v1.0.0 installed correctly (pip reported absentia-1.0.0) but absentia --version printed absentia 0.0.1 because src/absentia/__init__.py had __version__ = "0.0.1" hardcoded since the project's first commit. scripts/release.sh only bumps pyproject.toml. The two drifted on every release. Switched __version__ to read from importlib.metadata so pyproject.toml is now the single source of truth — no future drift possible.

[project.urls] for PyPI sidebar. Repository, Issues, Changelog, Documentation, Release Notes — all surface as clickable links on https://pypi.org/project/absentia/. v1.0.0 shipped with these empty.
README install rewrite. Two recommended paths, side by side:
```
uv tool install absentia       # Astral toolchain, ~10–100× faster
pipx install absentia          # the long-standing standard
```
Per-OS bootstrap tables for both. Plus a callout for the PEP 668 error: externally-managed-environment that modern Debian / Ubuntu / WSL / Fedora users hit on bare pip install — explains what's happening and points at uv tool / pipx / venv as the fixes.

uv tool install absentia
# or
pipx install absentia

Requires Python 3.13+. Cross-platform.