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.

README.md

@@ -63,8 +63,8 @@ See [src/board/PORTING.md](src/board/PORTING.md).
 
 ## Protocol
 
-See [PROTOCOL.md](https://github.com/donglora/protocol-rs/blob/main/PROTOCOL.md)
-in the [donglora-protocol](https://github.com/donglora/protocol-rs) repo
+See [PROTOCOL.md](https://git.swaits.com/donglora/protocol-rs/src/branch/main/PROTOCOL.md)
+in the [donglora-protocol](https://git.swaits.com/donglora/protocol-rs) repo
 (crates.io: [`donglora-protocol`](https://crates.io/crates/donglora-protocol))
 for the complete wire format specification — that is the canonical home;
 this firmware just consumes it.