docs(release): v3.15.0 — README AI prompt blocks + Upgrading rewrite + CHANGELOG

§Installation, §Upgrading, §Troubleshooting each start with a copy-paste
AI prompt block for Claude Code / Cursor / Copilot. The Upgrading section
explains the three paths (light / full / fresh-install) and rollback usage.

All Commands table gains an `ocp doctor` row.

package.json bumped to 3.15.0.

CHANGELOG.md gains the v3.15.0 entry covering doctor, the cross-version
update path, --rollback, fresh-install routing, and AI prompt blocks.
Notes the dependency on PR #90 (plist env merge bug fix, already merged).

No cli.js citation needed: docs + version bump only, no server.mjs change.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "open-claude-proxy",
-  "version": "3.14.0",
+  "version": "3.15.0",
   "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": {