bash: re-enable automatic bash shell detection

Bash shell detection was originally disabled in #1823 due to problems
with /bin/bash on macOS.

Apple distributes their own patched version of Bash 3.2 on macOS that
disables the POSIX-style $ENV-based startup path:

e5397a7e74/bash-3.2/shell.c (L1112-L1114)

This means we're unable to perform our automatic shell integration
sequence in this specific environment. Standard Bash 3.2 works fine.

Knowing this, we can re-enable bash shell detection by default unless
we're running "/bin/bash" on Darwin. We can safely assume that's the
unsupported Bash executable because /bin is non-writable on modern macOS
installations due to System Integrity Protection.

macOS users can either manually source our shell integration script
(which otherwise works fine with Apple's Bash) or install a standard
version of Bash from Homebrew or elsewhere.

src/shell-integration/README.md

@@ -18,9 +18,6 @@ our integration script (`bash/ghostty.bash`). This prevents Bash from loading
 its normal startup files, which becomes our script's responsibility (along with
 disabling POSIX mode).
 
-Because automatic Bash shell integration requires Bash version 4 or later, it
-must be explicitly enabled (`shell-integration = bash`).
-
 Bash shell integration can also be sourced manually from `bash/ghostty.bash`.
 This also works for older versions of Bash.
 
@@ -31,6 +28,13 @@ if [ -n "${GHOSTTY_RESOURCES_DIR}" ]; then
 fi
 ```
 
+> [!NOTE]
+>
+> The version of Bash distributed with macOS (`/bin/bash`) does not support
+> automatic shell integration. You'll need to manually source the shell
+> integration script (as shown above). You can also install a standard
+> version of Bash from Homebrew or elsewhere and set it as your shell.
+
 ### Elvish
 
 For [Elvish](https://elv.sh), `$GHOSTTY_RESOURCES_DIR/src/shell-integration`