Add Facebook Share and adopt master-only releases

[Kapusch]

What changed

• add selectable Facebook Share SDK interop alongside the existing Login integration;
• keep Login as the compatibility default and allow Login;Share for BloomFly;
• package only the native modules selected by the consumer;
• consolidate releases on master and require publishable commits/tags to be reachable from it;
• document repository structure, AI guidance, packaging and release operations.

Why

BloomFly needs Facebook Feed sharing without changing existing Login consumers. The repository also had package-publishing changes stranded on release/1.0.0; consolidating them into master makes the release history explicit and allows the obsolete release branch to be removed after merge.

Impact

Existing consumers remain on the Login-only default. BloomFly can opt into Login;Share. No release branch or historical tag is rewritten by this PR.

Validation

• native Android build completed;
• dotnet pack produced Kapusch.Facebook.Android.1.1.0.nupkg;
• package contents and selected native dependencies validated;
• workflow YAML and repository guidance validated.