chore(release): v3.13.0 — cache layer hardening (per-key isolation + bypass + chunked replay + singleflight) (#67)

Per release_kit overlay (CLAUDE.md § Iron Rule 5.5):
- package.json bumped 3.12.0 → 3.13.0
- CHANGELOG.md updated with v3.13.0 entry
- README.md § Response Cache updated to document the four hardening features

This is a release preparation commit. Tag push to v3.13.0 will trigger
.github/workflows/release.yml which auto-creates the GitHub Release.

cli.js does not perform proxy-layer response caching, stampede protection,
or replay; this release only ships internal cache-layer correctness and
concurrency improvements that do not change the OpenAI-compatible wire
surface visible to clients. No client-observable wire shape change.

Co-authored-by: dtzp555 <dtzp555@gmail.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "open-claude-proxy",
-  "version": "3.12.0",
+  "version": "3.13.0",
   "description": "OCP (Open Claude Proxy) — use your Claude Pro/Max subscription as an OpenAI-compatible API for any IDE. Works with Cline, OpenCode, Aider, Continue.dev, OpenClaw, and more.",
   "type": "module",
   "bin": {