feat(cache-magento): add stamp caching for vendor/ directory (#245)

Adds an opt-in `stamp` mode that caches the extracted `vendor/`
directory in addition to the Composer download cache. On a warm
hit, `composer install` is effectively a no-op and shaves 2–5
minutes off a full Magento install.

cache-magento/compute-stamp-paths.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Args: working_directory exclude_from_stamp
+# working_directory: absolute path (caller is responsible for realpath resolution)
+# exclude_from_stamp: newline-separated list of composer package names to exclude
+WORKING_DIR="$1"
+EXCLUDE_FROM_STAMP="${2:-}"
+VENDOR="${WORKING_DIR}/vendor"
+
+PATHS="${VENDOR}/**"$'\n'"!${VENDOR}/**/"$'\n'"!${VENDOR}/magento/magento2-base"$'\n'"!${VENDOR}/magento/magento2-base/**"$'\n'"!${VENDOR}/mage-os/magento2-base"$'\n'"!${VENDOR}/mage-os/magento2-base/**"
+
+while IFS= read -r pkg; do
+  pkg="${pkg#"${pkg%%[![:space:]]*}"}"
+  pkg="${pkg%"${pkg##*[![:space:]]}"}"
+  [[ -z "$pkg" ]] && continue
+  PATHS="${PATHS}"$'\n'"!${VENDOR}/${pkg}"$'\n'"!${VENDOR}/${pkg}/**"
+done <<< "$EXCLUDE_FROM_STAMP"
+
+echo "$PATHS"