[nexus] chore(01-02): install commit-msg hook and enable git rerere

- Add scripts/nexus-commit-msg-hook.sh (tracked source for hook)
- Install hook at .git/hooks/commit-msg (executable)
- Enable git rerere with autoupdate for automated conflict re-resolution

scripts/nexus-commit-msg-hook.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+# Nexus fork: enforce [nexus] prefix on all fork commits
+# Allows upstream merge commits and rebase-generated commits through
+MSG_FILE="$1"
+FIRST_LINE=$(head -1 "$MSG_FILE")
+
+# Skip merge commits (git generates these automatically during rebase/merge)
+if echo "$FIRST_LINE" | grep -qE "^Merge (branch|pull request|remote-tracking)"; then
+  exit 0
+fi
+
+# Skip fixup/squash commits (used during interactive rebase)
+if echo "$FIRST_LINE" | grep -qE "^(fixup|squash)!"; then
+  exit 0
+fi
+
+# Enforce [nexus] prefix
+if ! echo "$FIRST_LINE" | grep -qE "^\[nexus\]"; then
+  echo "ERROR: Commit message must start with [nexus]"
+  echo "  Got: $FIRST_LINE"
+  echo "  Example: [nexus] feat: add branding package"
+  exit 1
+fi