docs(readme): Affiliates Promo section (AgentRouter coupon) (#9194)

* docs(readme): add Affiliates Promo section (coupon for AgentRouter)

New collapsible <details> block under '🤝 Supported by our Open Source Friends',
labeld 'Affiliates Promo' to keep it visually and editorially separate from
actual sponsors. Sized at ~half (icon 32px, sub-tag text) so it does not
compete with the partner block above.

First entry: AgentRouter — $100 signup credit (per FREE_TIERS.md), free server
with higher latency, first-class support since v3.8.50. Models surfaced:
claude-opus-4-8, claude-opus-5, gpt-5.6-sol — with a live-list link to
agentrouter.org/v1/models so users can verify.

Clear caveat: 'Affiliate link — OmniRoute has no sponsorship or partnership
with this provider.' — and a footer inviting more coupons via issue.

* docs(readme): leave Affiliates Promo expanded by default

The block only has a single entry today; collapsing it would hide the
AgentRouter coupon from a casual skim. Add 'open' to <details> so the
content is visible on first load. Users can still collapse it manually.

* docs(readme): drop 'see live list at agentrouter.org/v1/models' sentence

---------

Co-authored-by: diegosouzapw <diegosouzapw@users.noreply.github.com>

README.md

@@ -261,6 +261,33 @@ curl http://localhost:20128/v1/chat/completions \
 
 <br/>
 
+<details open>
+<summary><sub><b>🎟️ Affiliates Promo</b> — free signup coupons from providers we don't sponsor (click to expand)</sub></summary>
+
+<sub><i>This section is for referral/coupon codes only. Sponsored partnerships live in <b>🤝 Supported by our Open Source Friends</b> above. OmniRoute has no sponsorship or partnership with the providers listed here — these are public coupons anyone can use.</i></sub>
+
+<table>
+  <tr>
+    <td align="center" width="120">
+      <a href="https://agentrouter.org/register?aff=70LM">
+        <img src="public/providers/agentrouter.png" width="32" alt="AgentRouter"/>
+      </a>
+      <br/><sub><b>AgentRouter</b></sub><br/><sub>agentrouter.org</sub>
+    </td>
+    <td>
+      <sub><b><a href="https://agentrouter.org/register?aff=70LM">AgentRouter</a></b> — affiliate signup · <b>$100 free credits</b> on signup (free server, expect higher latency — best for testing, not production). First-class support in OmniRoute since <b>v3.8.50</b>: Chat Completions, the Anthropic-compatible wire format and the OpenAI-compatible path. Available models include <code>claude-opus-4-8</code>, <code>claude-opus-5</code>, <code>gpt-5.6-sol</code> and more. <b><a href="https://agentrouter.org/register?aff=70LM">Grab your $100 →</a></b></sub>
+      <br/><br/>
+      <sub>⚠️ <i>Affiliate link — OmniRoute has no sponsorship or partnership with this provider.</i></sub>
+    </td>
+  </tr>
+</table>
+
+<sub>Know another provider with a generous free signup coupon that benefits OmniRoute users? Open an issue and we'll add it here.</sub>
+
+</details>
+
+<br/>
+
 <div align="center">
 
 ## 🎯 Combos — The Flagship