chore(release): v3.23.0 — sonnet-5 default, upgrade reliability, CLAUDE_SYSTEM_PROMPT, README restructure

Consolidates #167/#168/#170-#177 (merged since the v3.22.1 tag). 3.22.1 → 3.23.0.

Minor because #168 changes the default model for every request that omits
`model` (sonnet alias 4-6 → 5) and #175 makes CLAUDE_SYSTEM_PROMPT functional —
behavior changes and a newly-working env var; not major because no API surface
breaks and pinning restores the old default.

Release-kit walk: CLAUDE_SYSTEM_PROMPT README row added in #175 (incl. cache
caveat); Available Models table updated in #168 (sonnet-5 = default); no new
endpoint; models.json alias change is the SPOT edit; new docs/ files indexed in
Repository Layout (#172); bootstrap quirks retained in README §Troubleshooting.
Version sourced from package.json only (no stale refs — grepped).

Tag push v3.23.0 at this squash commit triggers release.yml.

Co-Authored-By: Claude <claude-opus-4-8> <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "open-claude-proxy",
-  "version": "3.22.1",
+  "version": "3.23.0",
   "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": {