chore(release): v3.20.0 — TUI billing-safety hardening for 2026-06-15 (PR-A/B/C, #137-139) (#140)

Bump 3.19.0 → 3.20.0 + CHANGELOG. Aggregates three already-reviewed, already-merged PRs:
- #137 (PR-A) TUI honesty/cache correctness
- #139 (PR-B) TUI concurrency limit + /health observability
- #138 (PR-C) 6/15 canary + flip/rollback runbooks + setup auth-probe guard

README env-var / endpoint tables were updated in the respective feature PRs.

Co-authored-by: dtzp555 <dtzp555@gmail.com>
Co-authored-by: Claude <claude-fable-5> <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "open-claude-proxy",
-  "version": "3.19.0",
+  "version": "3.20.0",
   "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": {