fix(deploy): validate declared manifest environment variables

[silviot]

Manifest declaration checks previously accepted the app’s top-level <name> or a <name> inside an XML comment, allowing undeclared deploy values that AppAPI silently drops. The shared helper now checks names within <environment-variables> after removing XML comments, with regressions for both false positives.

This PR follows the D-729 repository split: the helper and its tests remain at scripts/lib-exapp-register.sh and scripts/test-exapp-register.sh. The production --env driver changes and staging tests are now in https://github.com/codemyriad/systems/pull/11. Main has been merged and the deleted ops/deploy driver files are not restored.

Validation passed:

• bash -n scripts/lib-exapp-register.sh scripts/test-exapp-register.sh
• shellcheck -x scripts/lib-exapp-register.sh scripts/test-exapp-register.sh
• scripts/test-exapp-register.sh
• The systems PR’s full offline deploy regression suite against this checkout, including CLI forwarding, override precedence, secret protection, staging cleanup, and lifecycle failures.
• ./scripts/fold-changelog.sh --version 0.3.0-alpha.1 --check passes for all repository fragments.

Also corrects the missing heading and bullet in changelog.d/ci-recorder-selectivity.md so the full changelog validation succeeds.

No live deployment was performed.