feat(functions): add open-url and repo-open browser helpers

Extract the browser-detection and launch logic out of config-help into a
reusable open-url function, then build repo-open on top of it to open the
current repo's origin remote in a browser.

- open-url: resolve the best graphical browser (fish_help_browser -> BROWSER
  -> xdg-mime https handler -> known binaries -> xdg-open) and launch it
  backgrounded. Silent by default; -v/--verbose reports the browser, -s/--silent
  is accepted explicitly. Browser console chatter is discarded.
- repo-open: normalize the origin URL from HTTPS and SSH/scp forms, deep-link
  to the current branch (falling back to the remote default) and sub-directory.
  Provider layout resolved via git config browse.provider, hostname heuristic
  (github/gitlab/gitea/bitbucket, codeberg->gitea), then github default.
  -p/--print emits the URL, -r/--root ignores the sub-directory.
- config-help: replace the inlined browser block with an open-url call.
- abbr: add open-repo and url-open abbreviations that expand to the canonical
  command names on space/enter.
- docs: document both functions in the SSoT (fish-config.md + index) and add a
  repo-open row to the README doc-browsing table.

conf.d/abbr.fish

@@ -313,6 +313,12 @@ abbr -a ssct 'sudo systemctl status'
 abbr -a sscs 'sudo systemctl start'
 abbr -a sscr 'sudo systemctl restart'
 
+### Alternate command names ###
+# Expand to the canonical function name so muscle-memory typos still work,
+# while surfacing the real command instead of silently forwarding to it.
+abbr -a open-repo repo-open
+abbr -a url-open open-url
+
 ### History Expansions and Substitutions ###
 # Bash-style history expansion is opinionated (C3 overrides), gated atomically
 # with conf.d/tricks.fish, conf.d/puffer.fish, and functions/expand_*.fish.