Add preflight guard to release scripts

[eNkru]

What

Prefixes release:patch / release:minor / release:major with a new release:preflight script that fails fast unless:

• you are on main (release commits/tags must not land on a feature branch)
• the working tree has no uncommitted changes (otherwise unrelated dirty files get left behind or swept into the release commit)

Why

A release is a one-command chain (npm version → sync → check → commit → tag → push → CI builds & publishes). If it runs from the wrong branch or with a dirty tree, the tag goes out anyway and cleanup is manual. The guard makes those two mistakes impossible.

Tested

• ✅ clean tree on main → passes
• ✅ dirty tree → release: aborting, uncommitted changes: … (and release:patch stops before the version bump — package.json untouched)
• ✅ on a feature branch → release: aborting, not on main: <branch>