chore: repoint remaining links at git.swaits.com

Rewrites lingering github.com / *.sr.ht references to this Gitea instance,
translating GitHub and Sourcehut URL shapes to Gitea's (/blob/<ref>/ and
sr.ht /tree/<ref>/item/ -> /src/branch/<ref>/, /tree/<tag> -> /src/tag/<tag>,
/actions/workflows/X -> /actions?workflow=X, /edit/ -> /_edit/).

Left pointing at the old hosts on purpose: pull-request links (PRs were not
migrated), forks that were never migrated, and GitHub-only surfaces Gitea
lacks. OpenPGP identity proofs are untouched by design.

CONTRIBUTING.md

@@ -22,7 +22,7 @@ common tasks consume. No shared board code — each board is self-contained.
 
 ## Client Libraries
 
-Client libraries exist in [Python](https://github.com/donglora/client-py) and [Rust](https://github.com/donglora/client-rs).
+Client libraries exist in [Python](https://git.swaits.com/donglora/client-py) and [Rust](https://git.swaits.com/donglora/client-rs).
 To add a library in another language:
 
 1. Create a directory with the language's standard project layout