fix: keep nightly packaging consumer-specific

[vitormattos]

Refines the managed nightly template boundary after validating the first LibreSign production run.

• prepares the generic Nextcloud server before Makefile packaging so consumer Makefiles can use occ when needed;
• keeps generic Nextcloud package signing in the catalog instead of embedding LibreSign behavior;
• fixes literal \\n sequences in nightly release notes;
• adds ordering/regression coverage;
• bumps the workflow platform patch version to 0.6.1.

LibreSign-specific Makefile signing will be expressed as a consumer-local nightly-release.yml.patch after this catalog change is published.