chore: switch iManager dep to Packagist 2.0 stable

[bigin]

With bigins/imanager 2.0.0 published on Packagist, the workarounds the rewrite phase needed to consume the library from a sibling checkout are no longer necessary.

composer.json:

• Constraint bigins/imanager: 2.0.x-dev → ^2.0.
• repositories: [ {type: path, url: ../imanager, ...} ] removed.
• minimum-stability: dev + prefer-stable: true removed (Composer's stable default applies).

composer.lock regenerated against Packagist's 2.0.0 dist (231ca442cb00014d8c1f86d32e91e54a55f2ce08). Verified locally: vendor/bigins/imanager is now a real package directory, not a symlink, and the live ServBay-served install (https://scriptor.cms and /editor/) returns 200 on both routes against the new package copy.

Demo image:

• docker/Dockerfile drops the php /tmp/composer-rewrite.php step and the COPY that brought the helper in. composer.lock is no longer removed before install. Now: a plain composer install --no-dev --no-interaction --prefer-dist --optimize-autoloader.
• docker/composer-rewrite.php deleted — it existed solely to patch the repositories array at build time so a Packagist-less build could find iManager via a VCS repo. Verified end-to-end on a fresh clone: docker compose down -v && docker compose up -d --build boots both containers healthy, / and /editor/ return 200, and vendor/composer/installed.json confirms bigins/imanager 2.0.0 was pulled from Packagist (not from GitHub VCS, not from a sibling path).