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/termio/shell_integration.zig

@@ -1,4 +1,5 @@
 const std = @import("std");
+const builtin = @import("builtin");
 const Allocator = std.mem.Allocator;
 const ArenaAllocator = std.heap.ArenaAllocator;
 const EnvMap = std.process.EnvMap;
@@ -57,11 +58,21 @@ pub fn setup(
     };
 
     const result: ShellIntegration = shell: {
-        // For now, bash integration must be explicitly enabled via force_shell.
-        // Our automatic shell integration requires bash version 4 or later,
-        // and systems like macOS continue to ship bash version 3 by default.
-        // This approach avoids the cost of performing a runtime version check.
-        if (std.mem.eql(u8, "bash", exe) and force_shell == .bash) {
+        if (std.mem.eql(u8, "bash", exe)) {
+            // Apple distributes their own patched version of Bash 3.2
+            // on macOS that disables the ENV-based POSIX startup path.
+            // This means we're unable to perform our automatic shell
+            // integration sequence in this specific environment.
+            //
+            // If we're running "/bin/bash" on Darwin, we can assume
+            // we're using Apple's Bash because /bin is non-writable
+            // on modern macOS due to System Integrity Protection.
+            if (comptime builtin.target.isDarwin()) {
+                if (std.mem.eql(u8, "/bin/bash", command)) {
+                    return null;
+                }
+            }
+
             const new_command = try setupBash(
                 alloc_arena,
                 command,