refactor(hooks): port to POSIX sh

All three hooks changed shebang from bash to sh. No bashisms were in use
(no [[, no local, no arrays) so only the interpreter line moved. Verified
with sh -n, and dash smoke-run with a sample tool_input JSON.

hooks/assemble-agents.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # PostToolUse(Edit|Write) — auto-regenerate agent .md files.
 #
 # Trigger logic:

hooks/assemble-validate.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # PreToolUse(Bash) — validate all agent manifests before `git commit` in ~/.claude.
 #
 # Trigger: Bash command contains "git commit" AND current directory is under ~/.claude.

hooks/no-hand-edit-agents.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # PreToolUse(Edit|Write) — block hand-editing generated agent .md files.
 #
 # Generated files start with: <!-- GENERATED by _assembler ...