Send a User-Agent from the HTTP client (fixes Dev.to edge 403)

[DanMat]

Root cause

The plugin's CurlHttpClient set no User-Agent. PHP's curl sends none by default, and Dev.to's edge (Varnish) answers a no-UA request with an empty-bodied HTTP 403 before it ever reaches the Forem API. That is what was failing every blog_syndicate_post (external_id null every time), and why the error carried no upstream text (the empty body was the edge's, not Forem's).

Proven server-side against GET /api/users/me with the live key, from the danmat container:

UA=(none / PHP default)  -> 403   server: Varnish   body: (empty)
UA=curl/8.0              -> 200   server: Heroku    body: {"id":4111350,"username":"danmat"...}
UA=NimbusCMS-Blog/1.0    -> 200   server: Heroku    ...

Not the SVG (outbound body is clean markdown), not the key (valid, correct account), not a publish gate (a manual publish did not help, because the request never reached the API).

Fix

CurlHttpClient now sends a descriptive User-Agent (NimbusCMS-Blog/1.0, overridable). This also benefits Hashnode, which shares the client.
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