repo: move to github.com/tangency-labs/fieldcraft (URLs, updater, Pages)

The updater's release endpoint, docs, README, release notes and the Pages
URL point at the tangency-labs org. App IDs (io.github.jyviko.fieldcraft)
stay unchanged so installed copies keep their identity and settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

SECURITY.md

@@ -3,7 +3,7 @@
 FieldCraft is young; only `main` and the latest release get fixes.
 
 Please **don't open a public issue** for a security problem. Report it privately through the
-repository's [Security tab](https://github.com/jyviko/fieldcraft/security/advisories/new) with
+repository's [Security tab](https://github.com/tangency-labs/fieldcraft/security/advisories/new) with
 what you found, how to reproduce it (a model, deck or script), the version and platform, and the
 impact you expect.