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.

docs/fish-config.index

@@ -157,6 +157,10 @@ spark=### spark
 yt-dlp=### yt-dlp
 miscfns=## 5.14 Miscellaneous
 config-help=### config-help
+open-url=### open-url
+url-open=### open-url
+repo-open=### repo-open
+open-repo=### repo-open
 config-update=### config-update
 config-settings=### config-settings
 config-toggle=### config-settings