fix(install): per-file backup for shared hooks dir

$HOOKS_DIR ($HOME/.claude/hooks) is shared with other kits. Backing up
the whole directory cumulatively bloats foreign hooks across re-runs.
Switch to backup_file per KeiSeiKit-owned hook so only the 3 files we
actually overwrite get a .bak-TIMESTAMP sibling. backup_dir remains for
KeiSeiKit-owned directories (_blocks, _templates, _assembler, skills).

install.sh

@@ -92,6 +92,18 @@ backup_dir() {
   say "backed up existing $target to $backup"
 }
 
+# Per-file backup for shared directories like $HOOKS_DIR, where other kits
+# may drop sibling files we must not touch. Only the specific file is moved
+# aside to <file>.bak-TIMESTAMP.
+backup_file() {
+  local target="$1"
+  [ -f "$target" ] || return 0
+  local backup="${target}.bak-$(date +%s)"
+  mv "$target" "$backup"
+  BACKUP_PAIRS+=("$target|$backup")
+  say "backed up existing $target to $backup"
+}
+
 # Activate KeiSeiKit hooks by merging settings-snippet.json into the user's
 # settings.json. Idempotent:
 #   - If settings.json is absent, copy snippet verbatim (minus _comment key).
@@ -230,9 +242,12 @@ if [[ -f "$KIT_DIR/_assembler/.gitignore" ]]; then
 fi
 
 # --- copy hooks (refresh; hooks are logic, not config) ---------------------
+# $HOOKS_DIR is shared with other kits — back up each KeiSeiKit-owned hook
+# individually rather than the whole directory, so foreign hooks are not
+# dragged into .bak-TIMESTAMP snapshots on every re-run.
 say "copying hooks -> $HOOKS_DIR/"
-backup_dir "$HOOKS_DIR"
 for h in assemble-agents.sh assemble-validate.sh no-hand-edit-agents.sh; do
+  backup_file "$HOOKS_DIR/$h"
   cp -f "$KIT_DIR/hooks/$h" "$HOOKS_DIR/$h"
   chmod +x "$HOOKS_DIR/$h"
 done