fix: use portable shebang in plugin hooks

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

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

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