feat(proxy): local OpenAI-compatible proxy for OAuth providers (#25969)

Adds 'hermes proxy start' — a local HTTP server that lets external apps
(OpenViking, Karakeep, Open WebUI, ...) use a Hermes-managed provider
subscription as their LLM endpoint. The proxy attaches the user's real
OAuth-resolved credentials to each forwarded request, refreshing them
automatically; the client can send any bearer (it gets stripped).

Ships with one adapter — Nous Portal. The UpstreamAdapter ABC and
registry in hermes_cli/proxy/adapters/ are designed for additional
OAuth providers to plug in by name without server changes.

Commands:
  hermes proxy start [--provider nous] [--host 127.0.0.1] [--port 8645]
  hermes proxy status
  hermes proxy providers

Allowed Portal paths: /v1/chat/completions, /v1/completions,
/v1/embeddings, /v1/models. Anything else returns 404 with a clear
error pointing at the allowed list.

aiohttp is gated like gateway/platforms/api_server.py (try-import,
clean runtime error if missing). No new core dependency.

Tests: 24 unit tests + 1 separate E2E that spawns the real subprocess
and verifies the upstream receives the right bearer with the client's
header stripped.

website/sidebars.ts

@@ -96,6 +96,7 @@ const sidebars: SidebarsConfig = {
           items: [
             'user-guide/features/web-dashboard',
             'user-guide/features/extending-the-dashboard',
+            'user-guide/features/subscription-proxy',
           ],
         },
         {