feat(scripts): extract.sh — reverse sync diff/apply for brain templates

Plan simplification: dropped the broken middle elif heuristic that had
unbalanced shell expression. Kept clean two-step fallback: .template
suffix first, bare filename second.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/tests/extract-test.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -u
+SCRIPT_DIR="$(cd "$(dirname "$0")/.." && pwd)"
+source "$SCRIPT_DIR/lib/common.sh"
+
+FAILURES=0
+assert_eq() {
+    if [ "$1" = "$2" ]; then echo "PASS: $3"; else echo "FAIL: $3 (expected '$1', got '$2')"; FAILURES=$((FAILURES + 1)); fi
+}
+
+# Test: identical content → exit 0 (no diff)
+tmpdir=$(mktemp -d)
+brain="$tmpdir/brain"
+consumer="$tmpdir/consumer"
+mkdir -p "$brain/project-files" "$consumer"
+echo "same content" > "$brain/project-files/CLAUDE.md.template"
+echo "same content" > "$consumer/CLAUDE.md"
+
+BRAIN_ROOT="$brain" bash "$SCRIPT_DIR/extract.sh" --source="$consumer" --file=CLAUDE.md --dry-run >/dev/null 2>&1
+assert_eq "0" "$?" "identical content returns exit 0"
+
+# Test: different content → exit non-zero
+echo "consumer different" > "$consumer/CLAUDE.md"
+BRAIN_ROOT="$brain" bash "$SCRIPT_DIR/extract.sh" --source="$consumer" --file=CLAUDE.md --dry-run >/dev/null 2>&1
+result=$?
+[ "$result" -ne 0 ] && assert_eq "0" "0" "different content returns non-zero" || assert_eq "0" "1" "different content returns non-zero"
+
+rm -rf "$tmpdir"
+
+echo "---"
+echo "Failures: $FAILURES"
+exit $FAILURES