feat(config): add down-arrow binding for silent pj invocation on empty prompt

dot_config/fish/functions/_pj_bind_down.fish

@@ -0,0 +1,18 @@
+function _pj_bind_down --description "down arrow: pj on an idle empty prompt, else the preset down-or-search"
+    # Mirror of atuin's up binding (atuin owns up, this owns down). Everything
+    # that legitimately needs the arrow keys keeps them:
+    #   - non-empty buffer  → multiline cursor movement / forward history search
+    #   - paging mode       → tab-completion pager navigation (tab on an empty
+    #                         line opens the pager with an empty buffer)
+    #   - search mode       → history-search navigation
+    set -l buf (commandline | string collect)
+    if test -n "$buf"; or commandline --paging-mode; or commandline --search-mode
+        commandline -f down-or-search
+        return
+    end
+
+    # PJ_QUIET keeps pj silent when the tree has no package.json, so down is a
+    # no-op outside JS projects instead of printing an error on every press.
+    PJ_QUIET=1 pj
+    commandline -f repaint
+end