When running a shell for `--hold set the env variable KITTY_HOLD=1` to allow users to customize what happens

For instance the user could have

[ "$KITTY_HOLD" = "1" ] && exec kitten __hold_till_enter__

in their shell rc files to get back the previous Press enter or esc to
quit behavior.

docs/changelog.rst

@@ -68,6 +68,8 @@ Detailed list of changes
 
 - Fix a regression that caused changing :opt:`text_fg_override_threshold` or :opt:`text_composition_strategy` via config reload causing incorrect rendering (:iss:`6559`)
 
+- When running a shell for ``--hold`` set the env variable ``KITTY_HOLD=1`` to allow users to customize what happens (:disc:`6587`)
+
 
 0.29.2 [2023-07-27]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

docs/glossary.rst

@@ -223,3 +223,8 @@ Variables that kitty sets when running child programs
 
    Set when using the include directive in kitty.conf. Can take values:
    ``linux``, ``macos``, ``bsd``.
+
+.. envvar:: KITTY_HOLD
+
+   Set to ``1`` when kitty is running a shell because of the ``--hold`` flag. Can
+   be used to specialize shell behavior in the shell rc files as desired.

kitty/utils.py

@@ -1185,7 +1185,7 @@ def cmdline_for_hold(cmd: Sequence[str] = (), opts: Optional['Options'] = None)
     ksi = ' '.join(opts.shell_integration)
     import shlex
     shell = shlex.join(resolved_shell(opts))
-    return [kitten_exe(), 'run-shell', f'--shell={shell}', f'--shell-integration={ksi}'] + list(cmd)
+    return [kitten_exe(), 'run-shell', f'--shell={shell}', f'--shell-integration={ksi}', '--env=KITTY_HOLD=1'] + list(cmd)
 
 
 def safe_mtime(path: str) -> Optional[float]: