feat: default to 'show' subcommand when jjf is invoked without args

Mirrors the original bash+fzf script's behavior — `jjf` alone becomes
a quick rev browser. Implemented in shell wrappers (so the --emit
history-rewrite path still applies); the binary itself still prints
usage on bare invocation, for explicit invocations.

src/init/bash.sh

@@ -1,6 +1,10 @@
 jjf() {
+    # No args → default to `show` so `jjf` alone is a rev browser.
+    if [ $# -eq 0 ]; then
+        set -- show
+    fi
     # Meta-commands and bare flags bypass the picker and run the binary directly.
-    if [ $# -eq 0 ] || [ "$1" = "init" ] || [ "${1#-}" != "$1" ]; then
+    if [ "$1" = "init" ] || [ "${1#-}" != "$1" ]; then
         command jjf "$@"
         return $?
     fi

src/init/fish.fish

@@ -1,10 +1,10 @@
 function jjf
     set -e _jjf_pending_swap
-    # Meta-commands and bare flags bypass the picker and run the binary directly.
+    # No args → default to `show` so `jjf` alone is a rev browser.
     if test (count $argv) -eq 0
-        command jjf
-        return $status
+        set argv show
     end
+    # Meta-commands and bare flags bypass the picker and run the binary directly.
     if test "$argv[1]" = init; or string match -q -- '-*' $argv[1]
         command jjf $argv
         return $status

src/init/nu.nu

@@ -1,9 +1,7 @@
 def --env jjf [...args] {
+    # No args → default to `show` so `jjf` alone is a rev browser.
+    let args = if ($args | is-empty) { ["show"] } else { $args }
     # Meta-commands and bare flags bypass the picker and run the binary directly.
-    if ($args | is-empty) {
-        ^jjf
-        return
-    }
     let first = $args.0
     if $first == "init" or ($first | str starts-with "-") {
         ^jjf ...$args

src/init/zsh.sh

@@ -1,6 +1,10 @@
 jjf() {
+    # No args → default to `show` so `jjf` alone is a rev browser.
+    if [ $# -eq 0 ]; then
+        set -- show
+    fi
     # Meta-commands and bare flags bypass the picker and run the binary directly.
-    if [ $# -eq 0 ] || [ "$1" = "init" ] || [ "${1#-}" != "$1" ]; then
+    if [ "$1" = "init" ] || [ "${1#-}" != "$1" ]; then
         command jjf "$@"
         return $?
     fi