[yaunj]

Fixes #1133

Why

Environments overrides sync() and calls remove() for every environment not in the config, without checking this.additive. With additive_plugins: [environments], undeclared environments are still deleted, and nop mode plans the deletion instead of reporting it as suppressed.

Approach

• In Environments.sync(), skip remove() when this.additive is set.
• In nop mode, emit the same "Additive mode active: N deletion(s) suppressed by additive_plugins" NopCommand as Diffable.sync().
• Non-additive behaviour is unchanged.

Validation

• 2 new tests in test/unit/lib/plugins/environments.test.js (both fail without the fix).
• Unit suite: 1017 passed, 12 skipped.