Refactor: unify the two theme release configs into the shared config/release.js factory

[adekbadek]

Background

Release configuration currently lives in three places:

• config/release.js — a shared factory consumed by all 12 plugins (releaseConfig({ name, phpFile, npmPublish })).
• themes/newspack-theme/.releaserc.js — hand-rolled, covers the 6 classic theme variations.
• themes/newspack-block-theme/release.config.js — hand-rolled.

The two theme configs don't use the factory — they duplicate the entire skeleton: the branches array, the plugins chain (commit-analyzer, release-notes-generator, @semantic-release/npm, semantic-release-version-bump, and the @semantic-release/github block with the migration-era successComment/releasedLabels/failComment/failTitle: false flags), and the prepare ordering.

#179's predecessor PR (#186) already extracted the one piece that PR touched — the conditional @semantic-release/git commit step — into config/release-helpers.js (gitCommitStep()). That's "Layer A". This issue tracks "Layer B": folding the rest of the theme skeleton into the factory so there's a single source of truth.

Proposal

Generalize config/release.js so themes can call it too, parameterizing the parts that genuinely differ:

Aspect	plugins	newspack-theme	newspack-block-theme
version-bump files	[ phpFile ]	newspack-*/sass/theme-description.scss + newspack-*/style.css	src/scss/_theme-description.scss, functions.php, style.css, style-rtl.css
gitCommitStep assets	[ phpFile, 'CHANGELOG.md' ]	6× theme-description.scss + CHANGELOG.md	CHANGELOG.md, _theme-description.scss, functions.php
GitHub assets (zips)	single ./release/<name>.zip	6 theme zips	single zip
npmPublish	varies (only newspack-blocks publishes)	false	false
alpha channel	prerelease: true	prerelease: true	prerelease: 'alpha' ← one-off; reconcile or keep as a param

A factory signature like releaseConfig({ bumpFiles, gitAssets, githubAssets, npmPublish, alphaPrerelease }) collapses all three call sites to thin parameter objects.

Why a separate PR

This touches the release path for all 14 units (12 plugins + 2 theme configs covering 7 themes), so it warrants its own change with a full sandbox matrix run (alpha / hotfix / release-promotion across every unit), independent of the narrowly-scoped hotfix-guard fix in #186.

Notes

• Watch the block-theme prerelease: 'alpha' vs prerelease: true difference — decide whether that's intentional before unifying.
• The @semantic-release/github disabled-comment flags are migration-era (NPPM-2752 Phase 6); a unified factory is a good place to gate them in one spot when they're eventually re-enabled.

Follow-up to #186.