Fix release version lookup after dynamic versioning

[CollierKing]

Problem

The Check Version step in the release workflow fails:

Run set -eu
Error: Process completed with exit code 1.

0.4.0 made the package version dynamic — hatchling reads __version__ from the package — so there is no longer a version = line in pyproject.toml. The step's grep '^version = ' pyproject.toml matches nothing, and under set -eu a failed grep aborts the job.

This was an oversight in #28: the version's source changed without updating the one place that read it.

Fix

Read __version__ from the installed package, which is the same source hatchling builds the distribution from, and fail with an explicit message if it comes back empty.

pkg_name still comes from pyproject.toml, since name is static.

Testing

Ran the step's script verbatim against a synced checkout:

pkg-name=sqlalchemy-cloudflare-d1
version=0.4.0
tag would be: v0.4.0

v0.4.0 is what mark-release uses for the tag, matching the previous behaviour.