feat: respect pnpm minimumReleaseAge from pnpm-workspace.yaml

When invoked as the pnpm/pnpx shim, walk up from cwd looking for
pnpm-workspace.yaml (or a pnpm field in package.json) and use its
minimumReleaseAge and minimumReleaseAgeExclude as the floor and
exclusion list. Existing CLI args, env vars, and ~/.safe-chain/config.json
still override, so the project's pnpm config can act as a single source
of truth without forcing users to duplicate settings across two systems.

Fixes #460

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/safe-chain/src/config/packageManagerName.js

@@ -0,0 +1,23 @@
+/**
+ * Tracks the package manager name (e.g. "pnpm", "npm", "yarn") that this
+ * invocation is shimming. Stored as a small standalone module so that
+ * lightweight config readers can branch on it without transitively importing
+ * the heavy package-manager creator graph.
+ */
+
+/** @type {{name: string | null}} */
+const state = { name: null };
+
+/**
+ * @param {string | null} name
+ */
+export function setPackageManagerName(name) {
+  state.name = name;
+}
+
+/**
+ * @returns {string | null}
+ */
+export function getPackageManagerName() {
+  return state.name;
+}