Remote fetch checks now also recognize 404 as a non-fatal status

What

Two places that fetch remote content used to only special-case HTTP 403 (forbidden) responses when deciding whether a failure was fatal:

• The generic remote-config/answer-fetch helper, used for things like skill or answer retrieval, now also checks for HTTP 404 using a new omn check before treating the response as an unrecognized status failure.
• The egress probe used before fetching artifact content now also has a parallel 404 path (checking for a plain-text response with a "not found" reason), alongside its existing 403 (forbidden) check.

Why

Previously a 404 response in these paths could be treated as an unexpected failure. Now it is recognized as a normal, non-fatal outcome, similar to how 403 was already handled, so Claude Code can respond to a missing resource more gracefully instead of erroring out.