refactor: use direct Markdown subclass

[redtux]

Restore direct Markdown subclass; fix cold-import at the test boundary

Background

Plan 10 (collapse-impl-module) replaced the two-file __new__-redirect
architecture with a single-file abc.ABC + register() virtual-subclass
pattern, to fix a real circular import while also fixing the isinstance
breakage the old two-file split had introduced.

The traceback that motivated Plan 10's approach:

tests/test_spdx_markdown.py:12
  → src/spdx_markdown.py:20  from commitizen.changelog_formats.markdown import Markdown
  → .venv/.../changelog_formats/__init__.py:59  ep.name: ep.load()
  → importlib/metadata/__init__.py:181  functools.reduce(getattr, attrs, module)
  → AttributeError: partially initialized module has no attribute 'SPDXMarkdown'

This showed the cycle is real — but reproduction since then revealed
its scope is narrower than initially assumed.

Scope of the circular import

Two scenarios were tested with the direct-subclass version (no wrapper):

Approach A — direct subclass	Approach B — ABC wrapper
cz bump --dry-run --changelog — ✅ works	✅ works
Cold python -c "from ...spdx_markdown import SPDXMarkdown" — ❌ AttributeError	✅ works
Same cold import with import commitizen.changelog_formats first — ✅ works	✅ works
pytest collection — ❌ AttributeError (cold path)	✅ works (via conftest noneeded)
isinstance(formatter, SPDXMarkdown) — ✅ true (real inheritance)	✅ true (virtual subclass)

The cycle only manifests when our own plugin module is the first
thing in the process to import commitizen.changelog_formats. cz's
own CLI bootstrap always imports that package before scanning
third-party commitizen.changelog_format entry points, so production
usage was never actually at risk. pytest, run directly against the
test module, is the one cold-import path in our own toolchain.

Proposal

• Replace the abc.ABC / __new__ / register() wrapper in
  spdx_markdown.py with a plain class SPDXMarkdown(Markdown): —
  same __init__, get_metadata, and get_latest_full_release
  overrides as the current _SPDXMarkdownImpl, just renamed and made
  the only class in the module. Remove import abc, _resolve(),
  __new__, and SPDXMarkdown.register(...).
• Fix the cold-import at the one place it actually occurs — test
  collection — by adding a one-line warm-up import to
  tests/conftest.py:

  import commitizen.changelog_formats  # noqa: F401  — warm up before plugin import

• Mark docs/plans/10-collapse-impl.md as superseded with a
  one-line note linking to Plan 11.

Follow-up tasks from #4 / PR #5

Two issues were spotted in the CI and changelog after PR #5 was merged.
These are unrelated but small enough to include in this change:

• .github/workflows/publish.yaml — Reorder so GitHub Release
  creation happens before PyPI publish; switch from
  generate_release_notes: true to body_path: release-notes.md;
  quote tags glob pattern [v*] as ["v*"] for YAML compliance;
  fix indentation and remove stale comments.
• docs/changelog.md — Escape underscore in _impl.py so it
  renders correctly as literal text.

Acceptance criteria

• spdx_markdown.py contains one class, no wrapper, no register().
• isinstance(formatter, SPDXMarkdown) is True via real inheritance.
• tests/conftest.py contains the warm-up import.
• ruff check src/ — zero warnings, no noqa: E402 needed.
• uv run pytest -v — all tests pass unchanged.
• cz bump --dry-run --changelog succeeds.
• docs/plans/10-collapse-impl.md marked superseded with link to
  Plan 11.
• publish.yaml and changelog.md fixes applied.