docs: publish Composer-native PAM ecosystem

[DavidWilliamBalbino]

Summary

• describe PAM as the minimal runtime beneath independent Composer products
• switch official package names to the canonical pushinbr/pam-* namespace
• add a standard Start here flow to every product and package guide
• document distribution mirrors and migration-only compatibility repositories
• set PHP 8.5 as the default while retaining the explicit PHP 8.4 compatibility line

Verification

• node scripts/check-pam-syntax.mjs
• node scripts/check-repository-coverage.mjs
• git diff --check

The five legacy Composer packages remain resolvable and are now marked abandoned on Packagist with exact replacements.