From 2f7ef8cc53200a125ff64a891ed2baeca6ddd0be Mon Sep 17 00:00:00 2001
From: Benjamin Borbe <benjamin.borbe@gmail.com>
Date: Tue, 2 Jun 2026 11:46:45 +0200
Subject: [PATCH] feat(precommit): add check-index target to catch
 walker-output drift

PR #9 trimmed a doc that contained pre-canonicalized rule blocks but
shipped without running 'make build-index'. The 7 new agent-cmd/*
entries were silently absent from rules/index.json until PR #10 picked
them up retroactively. The lesson: walker-output drift is silent at
review time; rules/index.json passes JSON validation regardless.

Fix: add 'check-index' to the precommit chain. The target regenerates
the walker output to a tmp file, diffs against the committed index,
and fails with the diff + a 'run make build-index and commit' message
if they differ.

Behavior verified:
- Clean state (master HEAD c07b809): precommit passes
- Synthetic drift (extra line appended to rules/index.json): precommit
  fails with diff output and non-zero exit code

This closes the gap that turned a one-PR change (PR #9) into a two-PR
walker-regen catch-up (PR #10). Future trim/bootstrap PRs that touch
rule blocks will fail precommit locally if the walker output isn't
also committed.
---
 Makefile | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 87201c6..cc6ecab 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 SHELL := /bin/bash
 
 .PHONY: precommit
-precommit: check-links check-json
+precommit: check-links check-json check-index
 
 .PHONY: release-check
 release-check: precommit check-versions
@@ -26,3 +26,15 @@ check-versions:
 build-index:
 	@python3 scripts/build-index.py > rules/index.json
 	@echo "rules/index.json updated"
+
+.PHONY: check-index
+check-index:
+	@python3 scripts/build-index.py > /tmp/coding-rules-index-check.json
+	@if ! diff -q rules/index.json /tmp/coding-rules-index-check.json > /dev/null 2>&1; then \
+		echo "ERROR: rules/index.json is stale. Run 'make build-index' and commit the result."; \
+		diff -u rules/index.json /tmp/coding-rules-index-check.json | head -40; \
+		rm -f /tmp/coding-rules-index-check.json; \
+		exit 1; \
+	fi
+	@rm -f /tmp/coding-rules-index-check.json
+	@echo "rules/index.json up-to-date"