chore(chatgpt-gateway): add Deprecation/Sunset headers — retire to mcp.chitty.cc

[chitcommit]

Summary

Emit standard deprecation signals on the ChatGPT-specific MCP gateway. ChatGPT MCP doesn't require a unique hostname — the OpenAI spec only needs a valid OAuth issuer + MCP transport, both already on the canonical `mcp.chitty.cc` aggregator with broader scope coverage and unified audit.

Headers added (every response)

```
Deprecation: true
Sunset: Sat, 15 Aug 2026 00:00:00 GMT
Link: https://mcp.chitty.cc/mcp; rel="successor-version",
https://mcp.chitty.cc/sse; rel="alternate"
```

Retirement plan

1. Merge + deploy this PR → ChatGPT clients see deprecation signals.
2. Add `mcp.chitty.cc` to ChatGPT Connector config; OAuth flow auto-registers via `/register`.
3. Watch traffic shift from `mcp-chatgpt.chitty.cc` → `mcp.chitty.cc`.
4. After 2026-08-15: delete CF route `7de271ce55d64215b3d89f8e5c49309b`, delete `chittymcp-gateway` worker, delete `mcp-gateway- chatgpt/` subtree.

Companion PR

chittyos/chittyconnect#TBD also adds deprecation headers to `connect.chitty.cc/chatgpt/mcp` (same retirement target).

No tool/protocol behavior changes; pure response-header addition.

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📒 Files selected for processing (1)

• mcp-gateway- chatgpt/src/index.js

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Adversarial review request: evaluate security, policy bypass paths, and regression risk.

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