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

@@ -57,7 +57,7 @@ possible.
 
 ```lua
 {
-  "https://github.com/swaits/zellij-nav.nvim",
+  "https://git.swaits.com/swaits/zellij-nav.nvim",
   config = function()
     require("zellij-nav").setup()
 
@@ -74,7 +74,7 @@ possible.
 ### [vim-plug](https://github.com/junegunn/vim-plug)
 
 ```vim
-Plug 'https://github.com/swaits/zellij-nav.nvim'
+Plug 'https://git.swaits.com/swaits/zellij-nav.nvim'
 lua require("zellij-nav").setup()
 
 nnoremap <c-h> <cmd>ZellijNavigateLeft<cr>