feat(makefile): add deps, diff, lint targets and fzf-picker fallback

Ports useful quality-of-life targets from DeepSat's coding-rulesets
Makefile, adapted to this repo's two-scope (global + per-project)
structure.

New targets:
  make deps                       Install claude, jq, fzf, ripgrep,
                                  emacs via brew/apt/pacman. Idempotent
                                  (skips already-present tools). For
                                  new machines and VMs.

  make diff LANG=<lang> [PROJECT=<path>]
                                  Show unified diff between repo source
                                  and installed copies in a target
                                  project. CLAUDE.md excluded (seed-
                                  only, diverges by design).

  make lint                       Validate ruleset structure: top-level
                                  headings, 'Applies to:' headers on
                                  rule files, shebangs and exec bits on
                                  hook scripts.

Infrastructure:
  - Help migrated to awk-parsed ##@/## pattern; new targets document
    themselves via a single trailing `## ...` comment.
  - fzf-picker fallback: if PROJECT= is unset, install-lang and diff
    launch fzf over local .git dirs under $HOME. Keeps PROJECT=<path>
    for scripts/automation; only interactive users hit fzf.

  scripts/diff-lang.sh  Walks the file list the installer would copy,
                        diffs each against the target.
  scripts/lint.sh       Standalone ruleset structure validator.

1 files changed, 80 insertions, 0 deletions

diff --git a/scripts/diff-lang.sh b/scripts/diff-lang.sh
new file mode 100755
index 0000000..a72d2b9
--- /dev/null
+++ b/scripts/diff-lang.sh
@@ -0,0 +1,80 @@
+#!/usr/bin/env bash
+# Diff installed rulesets in a target project vs the repo source.
+# Usage: diff-lang.sh <language> <project-path>
+#
+# Walks every file the installer would copy and shows a unified diff for
+# any that differ. Files missing in the target are flagged separately.
+
+set -u
+
+LANG="${1:-}"
+PROJECT="${2:-}"
+
+if [ -z "$LANG" ] || [ -z "$PROJECT" ]; then
+  echo "Usage: $0 <language> <project-path>" >&2
+  exit 1
+fi
+
+REPO_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+SRC="$REPO_ROOT/languages/$LANG"
+
+[ -d "$SRC" ]     || { echo "ERROR: no ruleset for '$LANG'" >&2; exit 1; }
+[ -d "$PROJECT" ] || { echo "ERROR: project path does not exist: $PROJECT" >&2; exit 1; }
+PROJECT="$(cd "$PROJECT" && pwd)"
+
+changed=0
+missing=0
+
+compare_file() {
+  local src="$1" dst="$2"
+  if [ ! -f "$dst" ]; then
+    echo "MISSING: $dst"
+    missing=$((missing + 1))
+    return
+  fi
+  if ! diff -q "$src" "$dst" >/dev/null 2>&1; then
+    echo "--- $src"
+    echo "+++ $dst"
+    diff -u "$src" "$dst" | tail -n +3
+    echo
+    changed=$((changed + 1))
+  fi
+}
+
+echo "Comparing '$LANG' ruleset against $PROJECT"
+echo
+
+# Generic rules (claude-rules/*.md → .claude/rules/)
+for f in "$REPO_ROOT/claude-rules"/*.md; do
+  [ -f "$f" ] || continue
+  name="$(basename "$f")"
+  compare_file "$f" "$PROJECT/.claude/rules/$name"
+done
+
+# Language .claude/ tree
+if [ -d "$SRC/claude" ]; then
+  while IFS= read -r f; do
+    rel="${f#$SRC/claude/}"
+    compare_file "$f" "$PROJECT/.claude/$rel"
+  done < <(find "$SRC/claude" -type f)
+fi
+
+# CLAUDE.md is seed-only (install won't overwrite without FORCE=1), so skip it
+# in normal diff output. Users can diff it manually if curious.
+
+# githooks/
+if [ -d "$SRC/githooks" ]; then
+  while IFS= read -r f; do
+    rel="${f#$SRC/githooks/}"
+    compare_file "$f" "$PROJECT/githooks/$rel"
+  done < <(find "$SRC/githooks" -type f)
+fi
+
+echo "---"
+if [ "$changed" -eq 0 ] && [ "$missing" -eq 0 ]; then
+  echo "No differences."
+else
+  echo "Summary: $changed differ, $missing missing."
+  [ "$changed" -gt 0 ] && exit 1
+  [ "$missing" -gt 0 ] && exit 2
+fi