test: add build-time validation regression tests (Bash)

Bash equivalent of the PowerShell build-validation test, using a tar-based
repo copy per case to check the same four rejected-defect scenarios.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/test-build-validation.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+root=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd)
+work=$(mktemp -d)
+cleanup() { rm -rf -- "$work"; }
+trap cleanup EXIT
+
+new_repo_copy() {
+  local copy
+  copy=$(mktemp -d "$work/case-XXXXXX")
+  (cd "$root" && tar -cf - --exclude='.git' --exclude='generated' .) | (cd "$copy" && tar -xf -)
+  printf '%s' "$copy"
+}
+
+test_build_fails() {
+  local label=$1 mutate_fn=$2 copy
+  copy=$(new_repo_copy)
+  "$mutate_fn" "$copy"
+  if bash "$copy/scripts/build.sh" >/dev/null 2>&1; then
+    printf 'Expected build to fail for case "%s" but it succeeded.\n' "$label" >&2
+    exit 1
+  fi
+  rm -rf -- "$copy"
+}
+
+mutate_invalid_json() {
+  printf '} this is not json {' >> "$1/adapters/claude/config/settings.json"
+}
+test_build_fails 'invalid Claude settings.json' mutate_invalid_json
+
+mutate_duplicate_agent() {
+  sed -i 's/^name:.*/name: implementer/' "$1/shared/agents/architect/agent.yml"
+}
+test_build_fails 'duplicate agent name' mutate_duplicate_agent
+
+mutate_duplicate_plugin() {
+  local manifest="$1/adapters/plugins.tsv"
+  sed -n '2p' "$manifest" >> "$manifest"
+}
+test_build_fails 'duplicate plugin name' mutate_duplicate_plugin
+
+mutate_leftover_placeholder() {
+  printf '\n__NOT_A_REAL_PLACEHOLDER__\n' >> "$1/shared/global-instructions.md"
+}
+test_build_fails 'leftover template placeholder' mutate_leftover_placeholder
+
+printf 'PASS build validation: invalid JSON, duplicate agent, duplicate plugin, and leftover placeholders are all rejected\n'