When some tools spawn subshells, PROMPT_COMMAND may be inherited as an
environment variable while the __ghostty_hook function is not (bash
doesn't export functions by default). This causes "command not found"
errors on every prompt in the subshell.
Add 2>/dev/null to the __ghostty_hook entry in PROMPT_COMMAND so that it
silently no-ops in subshells where the function isn't defined. This also
silences any errors from inside __ghostty_hook itself, but those are all
terminal escape sequences and non-actionable.
See: #11245
4b9324f48a