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/subagent-driven-dev/go-fractals/scaffold.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Scaffold the Go Fractals test project
 # Usage: ./scaffold.sh /path/to/target/directory
 

tests/subagent-driven-dev/run-test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Run a subagent-driven-development test
 # Usage: ./run-test.sh <test-name> [--plugin-dir <path>]
 #

tests/subagent-driven-dev/svelte-todo/scaffold.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 # Scaffold the Svelte Todo test project
 # Usage: ./scaffold.sh /path/to/target/directory