fix: use portable shebang in shell scripts

Change #!/bin/bash to #!/usr/bin/env bash for better portability
on systems like NixOS where /bin/bash doesn't exist.

Fixes #910

packages/coding-agent/scripts/migrate-sessions.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Migrate sessions from ~/.pi/agent/*.jsonl to proper session directories.
 # This fixes sessions created by the bug in v0.30.0 where sessions were