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.

functions/config-help.fish

@@ -138,71 +138,8 @@ function config-help --description 'Open the offline fish shell configuration ma
 
         set -l page_url "file://$html_dir/$html_path"
 
-        # Browser detection — mirrors fish's help.fish priority order but
-        # resolves actual browser binaries before falling back to xdg-open.
-        # xdg-open dispatches on the file's MIME type (text/html), which can
-        # be associated with non-browser apps (e.g. ebook readers). Using a
-        # real browser binary directly with a file:// URI avoids that lookup.
-        set -l graphical_browsers \
-            firefox firefox-esr chromium chromium-browser google-chrome \
-            brave-browser vivaldi vivaldi-stable epiphany falkon qutebrowser \
-            opera x-www-browser htmlview
-
-        set -l browser $fish_help_browser
-
-        if not set -q browser[1]
-            if set -q BROWSER
-                echo $BROWSER | read -at browser
-                if not type -q $browser[1]
-                    set_color red
-                    echo "error: \$BROWSER '$browser[1]' is not a valid command" >&2
-                    set_color normal
-                    return 1
-                end
-            else
-                # Resolve the https scheme handler from xdg-mime and use its
-                # binary directly — most reliable on modern Linux desktops.
-                if type -q xdg-mime
-                    set -l desk (xdg-mime query default x-scheme-handler/https 2>/dev/null)
-                    if test -n "$desk"
-                        set -l candidate (string replace -r '\.desktop$' '' -- $desk)
-                        if type -q $candidate
-                            set browser $candidate
-                        end
-                    end
-                end
-
-                # Fall back to trying known browser binaries in order.
-                if not set -q browser[1]
-                    for b in $graphical_browsers
-                        if type -q -f $b
-                            set browser $b
-                            break
-                        end
-                    end
-                end
-
-                # Last resort: xdg-open (may hit wrong app for local files).
-                if not set -q browser[1]; and type -q xdg-open
-                    set browser xdg-open
-                end
-            end
-        end
-
-        if not set -q browser[1]
-            set_color red
-            echo "error: could not find a web browser — set \$fish_help_browser or \$BROWSER" >&2
-            set_color normal
-            return 1
-        end
-
-        set_color green
-        echo "Opening HTML docs in $browser[1]…"
-        set_color normal
-
-        # Background the browser so it doesn't block the terminal.
-        sh -c '("$@") &' -- $browser $page_url
-        return 0
+        open-url $page_url
+        return $status
     end
 
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