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_local/bin/executable_pj

@@ -14,6 +14,8 @@
 # Usage: pj [extra args…]   extra args are forwarded to the picked task
 #
 # Env:
+#   PJ_QUIET=1   suppress the "no task sources" error — the down-arrow
+#                bindings use this, where an empty tree is the normal case.
 #   PJ_STATE=FILE  key bindings pass a path; pj writes `open` to it when the
 #                  picker opens and `pj --run` writes `ran`, so the binding
 #                  can tell "no tasks" / "cancelled" / "task printed output"
@@ -390,7 +392,7 @@ while :; do
 done
 
 if [ ${#recs[@]} -eq 0 ]; then
-    echo "pj: no task sources found in this tree" >&2
+    [ -n "${PJ_QUIET-}" ] || echo "pj: no task sources found in this tree" >&2
     exit 1
 fi