fix: use portable shebang in plugin hooks

Replace #!/bin/bash with #!/usr/bin/env bash for better portability across systems

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

plugins/explanatory-output-style/hooks-handlers/session-start.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Output the explanatory mode instructions as additionalContext
 # This mimics the deprecated Explanatory output style

plugins/learning-output-style/hooks-handlers/session-start.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Output the learning mode instructions as additionalContext
 # This combines the unshipped Learning output style with explanatory functionality