fix(install-lang): refuse a colliding second bundle instead of clobbering

Installing a second language bundle into a project silently replaced the first one's config. settings.json and githooks are copied with cp -rT (always overwrite), so installing bash over elisp rewired the validate hook to validate-bash.sh and dropped check-parens from pre-commit, leaving validate-el.sh orphaned on disk. The output said [ok] for both. A project could lose its paren check or secret scan and read the install as success.

The guard detects which bundles a project already has, by the same rule fingerprint sync-language-bundle uses, and refuses when the incoming bundle would overwrite a file another one ships. It names each file at risk. FORCE=1 still overrides, and the message says that also re-seeds CLAUDE.md, which is destructive on a customized project.

Only three of the five shared filenames actually collide. gitignore-add.txt is appended and deduped, and CLAUDE.md is seed-only, so both compose across bundles already.

This doesn't decide whether polyglot projects are supported, and the guard shouldn't be read as "no". A non-overlapping pair still installs: bash ships settings.json and githooks with no coverage fragment, python ships only a coverage fragment, so the two compose today. The real line is overlap, not polyglot, and nothing chose it. The open question, along with the identical coverage target names both fragments define, is filed.

home reported this after scaffolding clock-panel with python and typescript, which hit the coverage fragment. The settings.json and githooks cases are worse and hadn't been noticed. No project was damaged: the three bundles that collide aren't doubled up anywhere.

1 files changed, 69 insertions, 0 deletions

diff --git a/scripts/install-lang.sh b/scripts/install-lang.sh
index 2f38fcd..6e8d806 100755
--- a/scripts/install-lang.sh
+++ b/scripts/install-lang.sh
@@ -33,6 +33,75 @@ fi
 # Resolve to absolute path
 PROJECT="$(cd "$PROJECT" && pwd)"
 
+# 0. Cross-bundle collision guard.
+#
+# Several bundles ship a file at the same path. gitignore-add.txt is appended
+# and deduped, and CLAUDE.md is seed-only, so both compose across bundles. Three
+# do not: claude/settings.json and githooks/* are installed with `cp -rT`
+# (always overwrite), and coverage-makefile.txt is seeded under a fixed name. So
+# installing a second bundle replaced the first's hook wiring and pre-commit
+# while printing [ok], so a project could lose its paren check or secret scan
+# and read the output as success. Refuse instead, naming what would go.
+#
+# Detection is by rule fingerprint, matching sync-language-bundle.sh: a project
+# has bundle X iff one of X's own rule files is in .claude/rules/. No marker
+# file, so this works on installs that predate the guard.
+project_has_bundle() {
+  local b="$1" rf
+  for rf in "$REPO_ROOT/languages/$b/claude/rules"/*.md; do
+    [ -f "$rf" ] || continue
+    [ -f "$PROJECT/.claude/rules/$(basename "$rf")" ] && return 0
+  done
+  return 1
+}
+
+# Files $1's bundle and $LANG both ship, and that install would overwrite.
+shared_overwritten_files() {
+  local other="$1" rel
+  for rel in claude/settings.json coverage-makefile.txt; do
+    [ -f "$SRC/$rel" ] && [ -f "$REPO_ROOT/languages/$other/$rel" ] \
+      && echo "  $(printf '%s' "$rel" | sed 's|^claude/|.claude/|')"
+  done
+  if [ -d "$SRC/githooks" ] && [ -d "$REPO_ROOT/languages/$other/githooks" ]; then
+    for rel in "$SRC/githooks"/*; do
+      [ -f "$rel" ] || continue
+      [ -f "$REPO_ROOT/languages/$other/githooks/$(basename "$rel")" ] \
+        && echo "  githooks/$(basename "$rel")"
+    done
+  fi
+}
+
+if [ "$FORCE" != "1" ]; then
+  collisions=""
+  for other_dir in "$REPO_ROOT/languages"/*/; do
+    other="$(basename "$other_dir")"
+    [ "$other" = "$LANG" ] && continue
+    [ -d "$other_dir/claude/rules" ] || continue
+    project_has_bundle "$other" || continue
+    files="$(shared_overwritten_files "$other")"
+    [ -n "$files" ] && collisions="${collisions}The '$other' bundle is already installed here. Installing '$LANG' would replace:
+${files}
+"
+  done
+
+  if [ -n "$collisions" ]; then
+    {
+      echo "ERROR: bundle collision. Refusing to install '$LANG' into $PROJECT"
+      echo
+      printf '%s' "$collisions"
+      echo "Both bundles ship these files, and installing overwrites rather than merges,"
+      echo "so the bundle already here would silently lose them."
+      echo
+      echo "Whether a project can carry two bundles at once is an open question."
+      echo "Until it's settled, install one bundle per project."
+      echo
+      echo "To override: re-run with FORCE=1. That also re-seeds CLAUDE.md from the"
+      echo "bundle template, which overwrites any project-specific edits to it."
+    } >&2
+    exit 1
+  fi
+fi
+
 echo "Installing '$LANG' ruleset into $PROJECT"
 
 # 1. Generic rules from claude-rules/ (shared across all languages)