fix(release): derive version from git tag via hatch-vcs (#1503)

[dimitri-yatsenko]

Resolves finding 1 of #1503. (Findings 2–4 already landed in #1504.)

Problem

The version bump PR lands after the release tag, so src/datajoint/version.py at each tag still reports the previous release (at the v2.3.1 tag it reads 2.3.0, and so on). PyPI wheels are unaffected — they're built from the sed'd tree in the same workflow run — but:

• pip install git+https://github.com/datajoint/datajoint-python.git@v2.3.1 → wheel reporting 2.3.0
• pip show datajoint on that install reports 2.3.0
• version-gated code (if datajoint.__version__ >= "2.3.1") silently takes the wrong branch

Fix — make the git tag the single source of truth (hatch-vcs)

• pyproject.toml: [tool.hatch.version] source = "vcs" (+ hatch-vcs build dep) and a [tool.hatch.build.hooks.vcs] hook that writes the resolved version to the generated, git-ignored src/datajoint/_version.py.
• src/datajoint/version.py: re-exports the generated version, falling back to installed package metadata (importlib.metadata) for source-tree runs — so datajoint.__version__ keeps working everywhere.
• .github/workflows/post_draft_release_published.yaml: drops the version.py sed/commit step and the follow-up bump PR entirely; checks out with fetch-depth: 0 so the build resolves the tag. Publishing the vX.Y.Z tag is now the only version action.
• .gitignore: ignore the build-generated _version.py.
• RELEASE_MEMO.md: document the tag-driven flow (no manual bump, no bump PR).

Verification

Built and installed locally against a throwaway v9.9.9 tag:

• hatchling version at the clean tag → 9.9.9; python -m build → datajoint-9.9.9-py3-none-any.whl and datajoint-9.9.9.tar.gz (the sdist→wheel path pip install git+…@vX uses).
• Installed wheel → datajoint.__version__ == 9.9.9 and importlib.metadata.version("datajoint") == 9.9.9.
• A dirty/untagged tree correctly yields a .devN+g<sha> version; _version.py is git-ignored and never tracked.

Notes

• No version.py value to bump by hand anymore; the tag sets the version for pip install, pip show, and datajoint.__version__ alike.
• Opened as a draft — this touches the release path, so worth a careful look before it's live. The workflow change can only be fully exercised on the next real release.