feat: add reusable Nextcloud release plan

[vitormattos]

Summary

Add the first non-mutating reusable workflow for Nextcloud app releases.

The planner validates:

• requested semantic version;
• stable branch;
• appinfo/info.xml version;
• changelog entry;
• optional milestone closure;
• caller-defined blocker queries for pending backports or other release blockers.

Architecture

The reusable workflow is limited to workflow-level orchestration: permissions, runner selection and checkouts.

Release planning is encapsulated by the actions/release-plan composite action. The action maps declared inputs to a small environment contract and invokes the tested Python planner. Parsing, release rules, GitHub checks, exit status and step-summary rendering remain in Python instead of inline workflow shell.

This keeps the shared workflow declarative while following GitHub's documented separation between reusable workflows and composite actions.

The release policy remains caller-owned: the shared workflow does not hard-code LibreSign labels or repository conventions.

This is the first implementation step for LibreCodeCoop/github-governance#5. Signing, artifact publication and App Store credentials remain outside the planning stage.