feat(ms-ai-architect): add lib/cross-platform-paths for cache/log/state/backup dirs [skip-docs]

First foundation lib for v1.12.0 auto-KB-update. Resolves per-OS paths:
- macOS: ~/Library/{Caches,Logs,Application Support}/<app>/
- Linux: XDG_CACHE_HOME / XDG_STATE_HOME with ~/.cache, ~/.local/state fallbacks
- Windows: %LOCALAPPDATA%\<app>\{Cache,Logs,State}

Plus getBackupDir(pluginRoot) → <pluginRoot>/.kb-backup (gitignored).

All four functions auto-mkdir target. Dependency-injection via opts
({platform, homedir, env}) makes the lib pure-testable; 13/13 tests
pass under tmpdir isolation without touching real ~/ paths.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/ms-ai-architect/scripts/kb-update/lib/cross-platform-paths.mjs

@@ -0,0 +1,105 @@
+// cross-platform-paths.mjs — Cache/Log/State/Backup dir resolution per OS.
+// Zero dependencies. macOS uses ~/Library/{Caches,Logs,Application Support},
+// Linux uses XDG with fallbacks, Windows uses %LOCALAPPDATA%.
+
+import { mkdirSync } from 'node:fs';
+import { homedir as osHomedir, platform as osPlatform } from 'node:os';
+import { join } from 'node:path';
+
+function resolveOpts(opts = {}) {
+  return {
+    platform: opts.platform ?? osPlatform(),
+    homedir: opts.homedir ?? osHomedir,
+    env: opts.env ?? process.env,
+  };
+}
+
+function ensureDir(path) {
+  mkdirSync(path, { recursive: true });
+  return path;
+}
+
+function requireApp(appName) {
+  if (!appName || typeof appName !== 'string') {
+    throw new Error('cross-platform-paths: appName is required and must be a non-empty string');
+  }
+}
+
+/**
+ * Resolve a per-app cache directory.
+ * @param {string} appName — application identifier, e.g. "ms-ai-architect"
+ * @param {object} [opts] — { platform, homedir, env } for testing
+ * @returns {string} absolute path to the cache directory (created if missing)
+ */
+export function getCacheDir(appName, opts) {
+  requireApp(appName);
+  const { platform, homedir, env } = resolveOpts(opts);
+  const home = homedir();
+
+  if (platform === 'darwin') {
+    return ensureDir(join(home, 'Library', 'Caches', appName));
+  }
+  if (platform === 'win32') {
+    const lad = env.LOCALAPPDATA || join(home, 'AppData', 'Local');
+    return ensureDir(join(lad, appName, 'Cache'));
+  }
+  // linux + everything else
+  const xdg = env.XDG_CACHE_HOME || join(home, '.cache');
+  return ensureDir(join(xdg, appName));
+}
+
+/**
+ * Resolve a per-app log directory.
+ * @param {string} appName
+ * @param {object} [opts]
+ * @returns {string} absolute path to the log directory (created if missing)
+ */
+export function getLogDir(appName, opts) {
+  requireApp(appName);
+  const { platform, homedir, env } = resolveOpts(opts);
+  const home = homedir();
+
+  if (platform === 'darwin') {
+    return ensureDir(join(home, 'Library', 'Logs', appName));
+  }
+  if (platform === 'win32') {
+    const lad = env.LOCALAPPDATA || join(home, 'AppData', 'Local');
+    return ensureDir(join(lad, appName, 'Logs'));
+  }
+  const xdg = env.XDG_STATE_HOME || join(home, '.local', 'state');
+  return ensureDir(join(xdg, appName, 'logs'));
+}
+
+/**
+ * Resolve a per-app state/data directory (persistent app state, not cache).
+ * @param {string} appName
+ * @param {object} [opts]
+ * @returns {string} absolute path (created if missing)
+ */
+export function getStateDir(appName, opts) {
+  requireApp(appName);
+  const { platform, homedir, env } = resolveOpts(opts);
+  const home = homedir();
+
+  if (platform === 'darwin') {
+    return ensureDir(join(home, 'Library', 'Application Support', appName));
+  }
+  if (platform === 'win32') {
+    const lad = env.LOCALAPPDATA || join(home, 'AppData', 'Local');
+    return ensureDir(join(lad, appName, 'State'));
+  }
+  const xdg = env.XDG_STATE_HOME || join(home, '.local', 'state');
+  return ensureDir(join(xdg, appName));
+}
+
+/**
+ * Resolve the backup directory under a plugin root.
+ * @param {string} pluginRoot — absolute path to the plugin root
+ * @returns {string} absolute path to <pluginRoot>/.kb-backup (created if missing)
+ */
+export function getBackupDir(pluginRoot) {
+  if (!pluginRoot || typeof pluginRoot !== 'string') {
+    throw new Error('cross-platform-paths: pluginRoot is required and must be a non-empty string');
+  }
+  return ensureDir(join(pluginRoot, '.kb-backup'));
+}