fish greeting: win over distro configs that define fish_greeting inline

On CachyOS (and OMF-style setups) config.fish sources a distro file that
defines fish_greeting while the shell starts. A function defined that way
beats autoloading from functions/, so the installed greeting never ran.

install.fish now appends a guarded block to ~/.config/fish/config.fish that
sources the greeting after everything else (idempotent via a marker, skipped
with --no-config), and the README documents the symptom, the check
(functions --details fish_greeting) and the manual one-liner.

Verified in a sandbox HOME that reproduces the CachyOS setup: before the
install fish resolves the distro file and prints its message, after it
resolves ~/.config/fish/functions/fish_greeting.fish and runs ours; a second
install leaves a single block. Applied to this machine's real config as
well, where fish_greeting now resolves to the user function and the
__j621_fetch_random helper is loaded.

extras/fish_greeting/install.fish

@@ -10,8 +10,9 @@
 #   fish install.fish --url https://j621.example.ts.net --token <api-token>
 #   fish install.fish --no-prompt          # never ask, keep/derive defaults
 #   fish install.fish --force              # rewrite an existing config
+#   fish install.fish --no-config          # do not touch ~/.config/fish/config.fish
 
-argparse 'url=' 'token=' 'no-prompt' 'force' 'help' -- $argv
+argparse 'url=' 'token=' 'no-prompt' 'force' 'no-config' 'help' -- $argv
 or begin
     echo "Try: fish install.fish --help"
     exit 1
@@ -43,6 +44,31 @@ mkdir -p $functions_dir
 cp "$here/fish_greeting.fish" "$functions_dir/fish_greeting.fish"
 echo "Installed $functions_dir/fish_greeting.fish"
 
+# --- Override distro/OMF greetings -------------------------------------------
+# Some setups (CachyOS, OMF themes, hand-rolled configs) define fish_greeting
+# inline while config.fish is being read. A function defined that way wins over
+# autoloading, so our file would never load. Source it from the end of
+# config.fish to define ours last.
+set -l marker "# >>> J621 greeting >>>"
+set -l user_config ~/.config/fish/config.fish
+if set -q _flag_no_config
+    echo "Skipping $user_config (--no-config)"
+else if test -f $user_config; and grep -qF "$marker" $user_config
+    echo "Config already sources the greeting (marker present)"
+else
+    mkdir -p (dirname $user_config)
+    begin
+        echo ""
+        echo "$marker"
+        echo "# Loaded after the distro config so it wins over an inline fish_greeting."
+        echo "if test -f $functions_dir/fish_greeting.fish"
+        echo "    source $functions_dir/fish_greeting.fish"
+        echo "end"
+        echo "# <<< J621 greeting <<<"
+    end >> $user_config
+    echo "Added the J621 greeting to $user_config (removes any distro greeting override)"
+end
+
 # --- Configuration ------------------------------------------------------------
 mkdir -p $config_dir