revert(shell): move iris integration to the iris branch

Reverts 43dfe48, 0692ae3, e0cdcd6 and the cable-description half of
098da70 (the star-prompt stats stay). master returns to the pre-iris
shell setup: atuin in fish/zsh/bash, the down -> pj binding with
PJ_QUIET, and the fish/zsh key bindings as they were. The iris work
lives on the `iris` branch.

dot_config/fish/functions/_pj_bind_down.fish

@@ -0,0 +1,21 @@
+function _pj_bind_down --description "down arrow: pj tv cable on an idle empty prompt, else fish's 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
+    # down-or-search is fish's own input function, called as a command exactly
+    # the way atuin's _atuin_bind_up calls up-or-search.
+    set -l buf (commandline | string collect)
+    if test -n "$buf"; or commandline --paging-mode; or commandline --search-mode
+        down-or-search
+        return
+    end
+
+    # _pj_bind runs ~/.local/bin/pj (also ctrl-p), which opens the `pj` tv
+    # cable. --quiet (PJ_QUIET) keeps it silent when the tree has no task
+    # source at all, so down is a no-op outside projects instead of printing
+    # an error per press.
+    _pj_bind --quiet
+end