init: OpenClaw Perf Cache — fs.{stat,lstat,realpath}{,Sync} TTL memo for plugin-tree paths

Wraps the global fs functions with a 1s TTL memo, scoped via path
whitelist to plugin-discovery paths only. Workaround for upstream
openclaw issue #86791: `loadPluginMetadataSnapshot()`'s cache-validity
check re-runs `hashWatchedFiles` on every lookup, which walks every
plugin's package.json + manifest + source via realpathSync ->
ancestor lstat chain. On prod t2 with ~100 plugins, one cache-check
pass is ~6 400 lstat + ~400 stat (~6-7s CPU per call). Fires on every
agent turn, every loadConfig() call, every channel routing decision.

This plugin doesn't fix the upstream design; it just absorbs the
repeated stats within a 1s window so the same paths aren't re-statted
6× per second during a discovery walk.

Verified on prod t2 (2026-05-27):
  - Cache hit ratio: 92.1-98.2% (stable across windows)
  - Idle baseline (0 turn, 0 push): 0.6-3.7% CPU (was 25%+ pre-fix)
  - Per-turn cost: notably reduced; previously 100% sustained per turn

Path whitelist:
  - /openclaw/dist/extensions/
  - /.openclaw/plugins/
  - /node_modules/@openclaw/
  - /openclaw/plugin-sdk/

All other paths pass through to original fs functions unchanged.

Manifest requires `activation.onStartup: true` so openclaw register()s
the plugin even though it exposes no tools/contracts (otherwise jiti
caches the module without ever calling register).

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

plugin/openclaw.plugin.json

@@ -0,0 +1,15 @@
+{
+  "id": "openclaw-perf-cache",
+  "name": "OpenClaw Perf Cache",
+  "version": "0.1.0",
+  "description": "Wraps fs.{statSync,lstatSync,realpathSync,promises.{stat,lstat,realpath}} with a short TTL memo for plugin-tree paths only. Workaround for upstream hashWatchedFiles being called on every plugin-metadata-snapshot cache-validity check (~800 statx/call × ~9 turns/hour on prod = sustained ~15-25% CPU baseline).",
+  "main": "index.js",
+  "activation": {
+    "onStartup": true
+  },
+  "configSchema": {
+    "type": "object",
+    "additionalProperties": false,
+    "properties": {}
+  }
+}