fix: use portable shebang #!/usr/bin/env bash in all shell scripts

Replace #!/bin/bash with #!/usr/bin/env bash in 13 scripts. The
hardcoded path fails on NixOS, FreeBSD, and macOS with Homebrew bash.
#!/usr/bin/env bash is the portable POSIX-friendly alternative.

Tested on Linux and Windows (Git Bash + CMD). macOS is the primary
beneficiary since Homebrew installs bash to /opt/homebrew/bin/bash.

Based on #700, closes #700.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/explicit-skill-requests/run-extended-multiturn-test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Extended multi-turn test with more conversation history
 # This tries to reproduce the failure by building more context