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.

.build.yml

@@ -1,11 +0,0 @@
-image: alpine/edge
-secrets:
-  - 88cce185-d2ec-4573-9546-e33f36f79bbf
-sources:
-  - git+ssh://git@git.sr.ht/~swaits/megapermute
-tasks:
-  - mirror-to-github: |
-      cd ~/megapermute
-      ssh-keyscan -t rsa github.com >> ~/.ssh/known_hosts
-      git remote add github git@github.com:swaits/megapermute.git
-      git push --mirror github

README.md

@@ -5,7 +5,7 @@ A million permutation hypothesis test utility.
 ## Installation
 
 1. If you don't already have rust installed, [install it](https://www.rust-lang.org/tools/install).
-2. Then run: `cargo install --git https://git.sr.ht/~swaits/megapermute`
+2. Then run: `cargo install --git https://git.swaits.com/swaits/megapermute`
 
 ## Usage